LED点阵的外文翻译

本科毕业设计（论文）外文翻译

学 生 姓 名： 黄杰 学 号： *********

二级学院名称： 电子信息学院 专 业： 电子科学与

技术

指 导 教 师： 徐方迁 职 称 副教授

合作/企业教师： 职 称

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（要求翻译与毕业设计（论文）相关的外文文献两篇，且3000单词以上/ 篇，将译文附

在原文之后）

第一篇：

[ 所译外文资料：

①作者：Taeil Jung

②书名（或论文题目）：Nano-structured InGaN Light-Emitting Diodes

for Solid-State Lighting

③出版社（或刊物名称或可获得地址）：All Rights Reserved.

④出版时间（或卷期号）：2009.

⑤所译起止页码：

Nano-structured InGaN Light-Emitting Diodesfor Solid-State Lighting

Solid-state lighting can potentially reduce the electricity consumption by

25%. It requires high efficiency light-emitting diodes across the visible

spectrum. GaN and related materials have direct band gap across the entire visible

spectrum and are ideal for future solid-state lighting applications. However,

materials defects, polarization charges, and total internal reflection have thus

far limited the efficiencies of InGaN LEDs, in particular InGaN LEDs in the

green/yellow wavelength range, which are critical in achieving highly efficient

LED luminaries with an excellent color-rendering index

In this Thesis, we have developed and demonstrated that novel in situ

nanostructured GaN processes in MOCVD are effective in improving the efficiencies

of InGaN LEDs. InGaN LEDs grown on quasi-planar semi-polar GaN templates were

proven to exhibit three times higher internal quantum efficiencies and negligible

quantum confined Stark effect using selective area epitaxy. InGaN LEDs grown on

nanostructured semi-polar GaN templates are also effective to improve the internal

quantum efficiency by 31%. The same in situ processes are also effective in

reducing the defect density by an order of magnitude and increasing the photon

extraction efficiency as a factor of two.

The in situ processes include in situ silane treatment and high temperature

overgrowth. Both processes require only standard MOCVD tools and hence are cost

effective and suitable for mass-production. In situ silane treatment treats

c-plane GaN samples with silane under ammonia environment, generating nano-scale

truncated cone structures with up to 200 nm scale. These truncated cone structures

can be subsequently transformed into pyramidal nanostructures comprising of only

(10-11) and (11-22) semipolar planes using high temperature overgrowth. These

processes were applied to both InGaN active region and the LED surface to improve

the internal quantum efficiency and the photon extraction efficiency,

respectively. Extensive materials, device, and optical characterizations have

been carried out in this research.

1.1 Gallium Nitride Materials for Optoelectronic Applications

Gallium nitride based materials, including GaN, AlN, InN, and their alloys,

are excellent candidates for short-wavelength optoelectronic applications. Their

direct bandgaps extend from ultraviolet to near-infrared. In addition, they

exhibit high mechanical and thermal stabilities compared to other III-V

semi-conductors, making them especially suitable for high-power and

high-temperature operations. In recent years, breakthroughs in p-type doping and

defect reduction have led to the commercialization of GaN based laser diodes,

light-emitting diodes (LEDs), high electron mobility transistors (HEMT) and

hydrogen detectors. Despite these advances, many technological challenges such

as green gap and substrate growths still remain.

Perhaps one of the most important applications for GaN based materials is

solidstate lighting (SSL). Worldwide, lighting constitutes 20% of electricity

consumption while its efficiency is much lower than 25%. In contrast, efficiency

of space heating has exceeded 90%. To this end, the development of highly efficient

and reliable LEDs for solid-state lighting has been very active in both industry

and academia in the past few years. It is projected by the US Department of Energy

that by 2015, if successful, solidstate lighting can reduce the overall

electricity consumption by 25%.

Unlike GaAs and InP based semi-conductors, GaN based materials have suffered

from a high density of defects due to very limited availability of lattice-matched

GaN substrates. Up to now, most GaN based optoelectronic devices have been

fabricated using hetero-epitaxy on foreign substrates such as sapphire (Al2O3),

silicon carbide (SiC), and aluminum nitride (AlN), and in a very small percentage

on silicon. Because of large lattice mismatch, GaN grown on these substrates often

exhibits a high density of threading dislocations, typically on the order of 108

– 1010 /cm2. These defects are still one of the major limiting factors for the

performance of GaN based optoelectronic devices, acting as non-radiative

recombination and scattering centers. Achievement of lower defect density would

also improve device reliability, resulting in a longer lifetime. Various defect

reduction approaches, such as epitaxial lateral over-growth (ELOG), have been

demonstrated and some of the details will be discussed in Chap.1.3.1. As part of

this thesis, we have explored a novel approach to using nano-structured GaN to

effectively lower the threading dislocation density.

Among various epitaxial techniques that have been developed for GaN based

materials, metal-organic chemical vapor deposition (MOCVD) is the leading

technology. The typical growth temperature for GaN materials is around 1000 to

1200°C. This high growth temperature is necessary to improve the crystal quality

and is a result of low cracking efficiency of the nitrogen source, ammonia (NH3),

at a low temperature. In Chapter 2, I will summarize my contributions to

successfully ramp up an MOCVD tool for the epitaxial growth of GaN LEDs for this

research.

1.2 InGaN LEDs for Solid-State Lighting

The basic component for SSL is a white-light LED. As shown in Figure 1-1, it

can be achieved by mixing various color components, which can be generated either

from the direct output of individual LEDs or from color-conversion materials, such

as phosphor. To date, commercially available white-light LEDs usually consist of

a blue emitter and a yellow phosphor plate. It has been shown that InGaN based

blue LEDs could achieve external quantum efficiency in excess of 70% [1, 2].

However, this di-chromatic configuration typically has a poor color rendering

index due to the lack of green and red components. The phosphor conversion process

also limits the overall luminous efficiency due to energy loss during

downconversion. To achieve luminous efficiency in excess of 200 lm/W and a color

rendering index (CRI) in excess of 90, which is required for general illumination,

a further improvement in blue LED efficiency and the use of tetra-chromatic

configuration (blue + green + yellow + red) is necessary [3].* Unfortunately, the

efficiency of both InGaN and AlInGaP LEDs decreases significantly in the

green-yellow (500 - 580 nm) range. This efficiency gap is also known as “green

gap”. Because AlInGaP materials have indirect bandgaps in this wavelength range,

to achieve high-efficiency SSL, it is crucial to significantly improve the

luminous

Note that a trichromatic (e.g. blue + green + red) source cannot achieve a

CRI > 90. efficiency of green and yellow InGaN LEDs. In this thesis, we will address

these challenges using nano-structured GaN.

Figure 1-1. Illustration of various potential white-light LEDs configurations

(after Ref. [4]).

1.3Limiting Factors for InGaN LEDs Efficiency

To date, the efficiencies of InGaN LEDs are still limited by materials defects,

polarization charges, and photon trapping. In this Section, we will briefly review

the state of theart and overview how this research helps address these limitations.

1.3.1 Materials Defects

As mentioned before, the high defect density in GaN based materials grown on

foreignsubstrates increases the non-radiative recombination rate and lowers the

radiative efficiency. To date, several techniques have been demonstrated to

improve the crystal quality and reduce the threading dislocation (TD) density of

the GaN layer. Substrate pretreatmentat the growth temperature in an ammonia

environment, also known as nitridation [5-7], has been shown to be critical for

high quality GaN epilayers. The TD density of a typical GaN layer grown on c-plane

sapphire substrate can be reduced to 108/cm2 [8] by employing the combination of

a low temperature (LT; 450 - 600 °C) nucleation layer (NL) and a short annealing

at the growth temperature to change the phase of the as-grown NL from cubic to

hexagonal [9-11]. As will be discussed in Chapter 2, careful optimization of these

low temperature growth sequences can significantly alter the subsequent GaN

template growth. To this end, a home-made optical in situ monitoring tool

(reflectometry) was established and will be discussed extensively in Chapter 2.

In addition low temperature buffer growth, epitaxial lateral overgrowth

(ELOG) which is a variation of selective area epitaxy (SAE) has been introduced

[12, 13] to further lower the TD density by an order of magnitude to below 107/cm2.

Variations of ELOG including pendeo- (from the Latin : hang on or suspended from)

epitaxy (PE) [14] and multi-step ELOG are also effective to further reduce the

TD density. Additional techniques such as TiN nano-porous network [15] and anodic

aluminum oxide nano-mask [16] have also been proposed and demonstrated. All these

methods, however, require ex situ processing and hence will add complexity and

cost to the manufacturing. In this thesis, we will explore and generalize an in

situ silane treatment approach to effectively lowering the TD density by an order

of magnitude.

1.3.2 Polarization Charges

Due to the non-cubic symmetry of GaN materials, compressively-strained active

regions in InGaN LEDs exhibit both spontaneous and piezoelectric polarization

charges. These polarization charges induce a strong internal electric field (IEF),

typically on the order of MV/cm, in the active region, resulting in both efficiency

droop at a high injection current density and the decrease of radiative efficiency

with an increasing emission wavelength. The IEF can separate electrons from holes

and increase electron leakage, resulting in low internal quantum efficiency (IQE)

and efficiency droop [17], respectively. The suppression of the IEF, which is

expected to increase IQE and the current density at which efficiency droop occurs,

can be achieved by reducing the lattice mismatch in hetero-structures or growing

them on semi-polar (e.g. {10-11} and {11-22}) and non-polar (e.g. a-plane and

m-plane) surfaces. Because indium incorporation is more difficult on non-polar

planes than on semi-polar planes, it is more advantageous to fabricate

long-wavelength green-yellow LEDs on semi-polar planes to suppress the IEF.

At least three approaches to fabricating semi-polar InGaN LEDs have been

reported thus far. These include the growth of a GaN epilayer on spinel substrates

[18], on bulk GaN substrates [19-27], and on the sidewalls of pyramidal or ridge

GaN structures created on planar polar GaN surfaces using SAE [28-35]. GaN grown

on spinel substrates have so far exhibited a high density of threading dislocations

and stacking faults, thereby compromising the potential improvement of efficiency

from the lowering of IEF. The use of bulk semi-polar GaN substrates has

demonstrated the advantage of a lower IEF for the enhanced efficiency of green

and yellow LEDs [25, 26]. However, limitations such as prohibitively high wafer

cost and small substrate size need to be resolved before this approach can become

more practical. On the other hand, the SAE technique can create semi-polar planes

on polar GaN surfaces.

High quality polar GaN films have been fabricated from a variety of substrates

including sapphire, 6H-SiC, and bulk GaN by MOCVD. Using growth rate anisotropy

and three-dimensional growth, different semi-polar and non-polar GaN planes can

be generated on c-plane GaN [13]. In Chapter 3, we will show that high quality

InGaN multiple quantum wells (MQWs) which exhibit IQE as large as a factor of three

compared to polar MQWs can be grown on pyramidal GaN microstructures. This

approach, however, requires ex situ patterning processes and does not easily

produce a planar structure for electrical contacts. In this thesis, a new

semi-polar LED structure is investigated, which is enabled by a novel epitaxial

nanostructure, namely the nanostructured semi-polar (NSSP) GaN, which can be

fabricated directly on c-plane GaN but without the issues of the SAE technique

mentioned above [36]. NSSP GaN also eliminates the issues of excessive defects

for GaN grown on spinel substrates and lowers the cost of using bulk semi-polar

GaN substrates. As we will show later, the surface of NSSP GaN consists of two

different semi-polar planes: (10-11) and (11-22). Therefore it is expected that

InGaN active regions fabricated on NSSP GaN can exhibit a low IEF, and hence much

improved IQE.

1.3.3 Photon Extraction

After photons are generated from the active region in LEDs, they need to escape

the device in order to be useful. When light travels from a medium with a higher

refractive index to a medium with a lower refractive index, total internal

reflection (TIR) occurs at the interface. In InGaN LEDs, photons experiencing TIR

at LED surfaces can be re-absorbed by the active region or trapped in the device

due to a wave-guiding effect as shown in Figure 1-2. In a simple InGaN LED, only

4% of photons generated from the active region can escape from each device surface.

It has been shown that surface textures on LED surfaces can greatly reduce TIR

and improve photon extraction efficiency as illustrated in Figure 1-2. To date,

many surface texturing techniques such as photonic crystal structures [37] and

photo-electrochemical etching of GaN surfaces [38] have been introduced. Notably,

the photo-electrochemical etching of nitrogen-terminated GaN surface has been

successfully implemented into commercial blue LEDs [2]. However, these approaches

all require additional ex situ patterning processes which add significant costs.

In this thesis, we investigate an in situ process to fabricate nano-structured

GaN surfaces on LEDs which effectively improves the photon extraction

efficiency. Figure 1-2. Light traveling within waveguides (left) with a smooth

interface and (right) with a rough interface (after [39]).

1.4 Organization of the Thesis

The objective of this thesis is to investigate cost-effective nanofabrication

techniques that can significantly improve the efficiency of the state-of-the-art

InGaN LEDs in both blue and green/yellow ranges for high performance solid-state

lighting. The organization of this thesis is as follows.

In Chapter 2, a summary of the MOCVD techniques for InGaN LEDs is given. In

Chapter 3, we study the dependence of InGaN LED IQE on {10-11} semi-polar planes

using SAE. In Chapter 4, fabrication and characterization of novel and

cost-effective nano-structured GaN templates will be described. Using in situ

silane treatment (ISST) and high temperature overgrowth (HTO), the formation of

nano-scale inverted cone structures and nano-structured semi-polar (NSSP)

templates has been obtained. In Chapter 5, we study InGaN semi-polar LEDs based

on NSSP templates. An improvement of internal quantum efficiency is demonstrated.

A green semi-polar InGaN LED grown on a c-plane substrate is also demonstrated.

In Chapter 6, current spreading in NSSP InGaN LEDs will be discussed. In Chapter

7, the application of ISST for the

improvement of photon extraction efficiency of an InGaN LED will be discussed.

In Chapter 8, we will summarize and make suggestions for future work.

2.1 Gallium Nitride Growth

As mentioned in the Introduction, gallium nitride (GaN) and related alloys

are excellent candidates for future solid-state lighting. To date, III-nitride

epitaxial growth has been limited by the lack of sufficiently large single crystal

substrate for homoepitaxial growth. Therefore, the growth of GaN and related

materials has been largely based on hetero-epitaxy using hydride vapor phase

epitaxy (HVPE), metal organic chemical vapor deposition (MOCVD), and molecular

beam epitaxy (MBE). Among these techniques, MOCVD is the leading technology due

to the advantages on material quality, scalability, and cost [40]. The material

quality of GaN grown by MOCVD has been excellent owing to its relatively high growth

temperature (1000 - 1200°C) [41, 42].

To date, various substrate materials including sapphire (Al2O3), silicon

carbide (SiC), and silicon have been studied for GaN growth (Table 2-1). Although

GaN substrates have been recently introduced in markets through bulk material

growth on foreign substrates using HVPE and laser cutting along specific crystal

planes, the cost has been prohibitively high. On the other hand, GaN grown on

c-plane (0001) sapphire substrate exhibits stable growth over a wide range of

growth conditions despite high dislocation density at the interface between the

substrate and epitaxial layer. In this research, I have helped ramping up an MOCVD

system together with Dr. Hongbo Yu. In this Chapter, I will summarize the MOCVD

technologies and defect reduction strategies for InGaN light-emitting diodes

(LEDs) epitaxy that will be used throughout this Thesis.

2.1.1 GaN Growth Using MOCVD

Due to a large lattice mismatch between GaN and sapphire, it is important to

contain the defects near the GaN/sapphire interface such that the defect density

can be minimized in the device region. Such optimization is achieved using in situ

reflectometry [44, 45]. A home-made reflectometry system shown in Figure 2-1 was

established in our 3 x 2” Thomas-Swan Close-Coupled Showerhead (CCS) MOCVD

system. White light is reflected from the sample surface and monitored by a

spectrometer during the growth. The reflectivity is sensitive to both the

surface morphology and the epitaxial layer structure.

Figure 2-1. Illustration of a home-made in situ reflectometry system

integrated into the MOCVD system.

Figure 2-2. Typical growth conditions for GaN templates used in this research.

Typical growth conditions for GaN templates used in this research are

summarized in Figure 2-2 and Table 2-2. Unless otherwise mentioned, c-plane

sapphire substrates were used. The five steps outlined in Table 2-2, including

high temperature (HT) cleaning, nitridation, low temperature (LT) nucleation,

annealing of LT nucleation layer, and HT GaN growth, are crucial for high quality

GaN epilayer.

Figure 2-3 and Table 2-3 show the corresponding in situ reflectometry signal.

In the following, we will describe how the reflectometry signal can be used to

optimize the GaN template growth. Unless otherwise mentioned, we will refer to

the reflectometry signal shown in Figure 2-3.

Figure 2-3. In situ reflectometry trace of GaN template growth (Sample ID :

UM-S07- 254). The highlighted areas correspond to important sub-steps during the

epitaxy.

2.1.1.1 High Temperature Cleaning

Initially, as the sample temperature is ramped up, the reflectivity increases

due to the increase of the refractive index of the sample. Kim et al. has thoroughly

studied the effect of initial thermal cleaning on the sapphire substrate and

experimentally demonstrated that this thermal treatment can effectively reduce

the surface roughness of the substrate [46]. Generally, the flat surface is

preferred for the GaN nuclei to be formed uniformly, which is critical to the

crystal quality of the final GaN epilayer. The specific condition for the HT

cleaning should be optimized by examining the treatment temperature and time. In

our GaN growth, the optimal treatment temperature and time were set to be 1075 °C

and 5 minutes, respectively. Moreover, HT surface annealing can effectively

eliminate surface moisture.

2.1.1.2 Nitridation

Nitridation [5, 7] is the process of NH3 preflow under hydrogen (H2) ambient

to prepare the surface for growth. During nitridation, NH3 reacts with the surface

oxygen atoms on the sapphire substrate. Due to the replacement of the oxygen atoms

by the nitrogen atoms and the diffusion of the nitrogen atoms into a certain depth,

the exposed surface becomes a smooth amorphous state. Because this change of

surface morphology is on the order of tens of angstrom, the corresponding

reflectivity change is not significant. It has been shown that with a proper

nitridation condition, GaN epilayers with lower dislocation density and better

electrical and optical properties can be achieved [7]. However, as mentioned

above, suitable combination of reactor conditions such as temperature, treatment

time, and NH3 flow rate must be considered. In our GaN growth, the nitridation

was optimized at 530 °C for a total of 210 seconds under 3 slm of NH3 flow.

2.1.1.3 Low Temperature Nucleation

As mentioned in Section 1.3.1, several approaches have been introduced to

reduce the threading dislocation (TD) density in growing the GaN template.

Specifically, the use of low temperature nucleation layer (LT NL) has been shown

to be simple yet effective. A threading dislocation density as low as 108/cm2 has

been reported [8].

As GaN is nucleated on sapphire, the cubic phase islands are first formed at

a temperature of 450 - 600 °C. These islands are subsequently transformed into

the wurtzite phase [8]. The increase of the reflectivity during the LT NL growth

is attributed to the increase of reflection from the flat top surfaces of nuclei.

Basically, we know that the reflection from GaN is about twice stronger than that

from sapphire due to the difference in refractive indices. As the islands become

denser (i.e. the growth time of LT NL becomes longer), total reflection from the

top surface of nuclei becomes up to 200% of reflection from sapphire substrate

assuming that the entire surface is covered by GaN islands. Even though the islands

are not coalesced completely to form a crystalline layer, this is still possible

because the distances between the adjacent islands are too small compared to the

optical wavelength. Once the reflectance exceeds twice that of the sapphire (as

shown in Figure 2-3), the islands continue to coalesce further, which results in

larger GaN grains and a thicker NL. Here, the size of the nucleation islands and

the thickness of the NL are critical to obtain high quality GaN epilayer. To show

that, we have compared a series of GaN templates with different NL conditions.

All conditions were kept the same† except the growth time of the LT NL was varied,

resulting in different LT NL thicknesses. The thickness of the LT NL was

extrapolated by analyzing the reflectometry data as the reflection ratio at the

end of LT NL growth to the sapphire substrate (RLT NL / RSapphire). The qualities

of the GaN templates were characterized using photoluminescence (PL) and x-ray

diffraction (XRD). From these results, the best GaN template quality can be

obtained when RLT NL / RSapphire is around 2.6 which corresponds to a 40nm thick

NL, at the given growth conditions.

† LT NL growth temperature = 530°C, V/III = 9140, LT NL annealing time = 420

seconds, HT GaN growth temperature = 1040°C, V/III = 1230, growth time = 4300

seconds.

Figure 2-4. The comparison of GaN template qualities with respect to the

reflection ratio between the LT NL surface and the sapphire substrate.

2.1.1.4 Annealing of Low Temperature Nucleation Layer

In GaN hetero-epitaxy with a large lattice mismatch, the initial growth on

the surface follows the Volmer Weber model [47], i.e. GaN island growth dominates.

In order to obtain smooth GaN templates, these islands need to be transformed into

the layer-by-layer growth mode using an NL annealing process. During annealing,

the substrate temperature is gradually increased up to around 1030 - 1050 °C under

NH3 overpressure. Temperature ramping rate, reactor pressure, and NH3 flow can

control the NL decomposition rate, which determines the surface roughness at the

end of the annealing process [48, 49]. In Figure 2-3, after point (h) at which

LT NL annealing begins, slight increase of reflectance is normally observed. The

increase continues until around 800 °C at which GaN decomposition process starts

to occur. Once the reflection intensity peaks, it begins to drop due to the increase

in surface roughness. Initially randomly distributed islands start to be

transformed into relatively uniform islands due to the decomposition of the NL

and the migration of the gallium ad-atoms.

During the annealing process, the reflectivity first decreases due to the

increase of surface roughness. Further annealing results in a slight increase of

reflectivity because at a higher temperature, the surface morphology becomes

smoother. However, if we anneal the surface even further, the surface roughness

increases again, which results in the decrease of reflection intensity [48, 49].

This phenomenon can be explained by considering the volume of the GaN islands.

At the transition point ((k) in Figure 2-3), the volume of the islands per unit

area becomes the highest which is preferable for the subsequent HT GaN growth.

As a rule of thumb, the position of this (reflectometry trace) shoulder is

dominated by the highest temperature of the annealing process [50]. In summary,

the goal of the low temperature nucleation and the subsequent annealing is to

achieve a surface morphology with proper density and sizes of the islands for the

following HT GaN growth.

As shown in Figure 2-5, even a slight change of the island distribution caused

by a slight difference of the NL thickness and temperature ramping rate (Table

2-4) can result in a significant difference in the following HT GaN growth under

the same conditions. In general, it takes longer for an NL with a rougher surface

and smaller islands to be transformed into the 2D growth mode. The conditions to

achieve high crystal quality GaN on sapphire are mostly related to the growth and

annealing of the LT NL.

2.1.1.5 HT GaN Growth

As soon as the sapphire surface is covered with suitable volume, uniformity,

thickness, and density of GaN islands, HT GaN growth can be followed. This HT GaN

itself can be divided into two parts (Figure 2-6). Part I corresponds to the initial

stage of HT GaN growth when the growth mode is transitioned from 3D to 2D, which

affects the crystal quality significantly. In part II, GaN epilayer becomes

thicker because the growth mode as well as growth condition is stabilized for 2D

mode. Several strategies to control the GaN growth in each regime will be briefly

discussed in the following.

The growth in part I is a buffer step to prepare a surface suitable for HT

GaN growth. During this step, the oscillation of the reflectometry signal becomes

increasingly obvious. Initially, the reflectivity continues to drop due to the

increase of surface roughness induced by the coagulations of the islands, i.e.

3D growth. As time goes by, the 3D growth mode is suppressed and the 2D growth

mode is enhanced. Once the surface becomes flattened due to the enhanced 2D growth,

layer by layer growth of GaN begins, which causes the reflectivity to increase.

The duration of this part of growth can be optimized by tweaking the reactor

pressure, V/III ratio, and growth rate [51, 52]. For example, in the case of a

low V/III ratio, it takes longer to recover the reflection intensity, which implies

that the change of the growth mode (3D 2D) occurs more slowly. The reflectivity

recovery time is critical to oscillation amplitude in part II. In general, a larger

oscillation amplitude corresponds to a better crystal quality.

The part II of the HT GaN growth is stable in a wide range of growth conditions

because the growth occurs in a mass transfer limited region. Nevertheless, several

key factors will still affect the crystalline structure, including the growth

temperature, trimethyl-gallium (TMG) flow, NH3 flow, V/III ratio, and reactor

pressure. As shown in Figure 2-7, the growth rate increases as the group III flow

increases but decreases as the V/III ratio and growth temperature increase. The

growth rate is one of the key parameters to determine optical and electrical

properties of GaN epilayer especially for p- or n- type doped cases. This will

be discussed in more details in the next Section.

译成中文：

纳米结构InGaN发光Diodesfor固态照明

固态照明用电可能减少25%。它需要高效率的发光二极管在可见光谱。赣江和相关的材

料直接带隙。整个可见光谱和未来固态照明的理想应用。然而,材料缺陷,极化费用和内部

反射总量迄今为止有限的功效InGaN LED,尤其是InGaN LED在绿色/黄色波长范围,从而

实现高效关键LED灯具与一个优秀的显色指数

在这篇论文中,我们已经开发出并证实小说原位纳米敢过程是有效的在MOCVD提高效率

InGaN发光二极管。InGaN种植在quasi-planar semi-polar led赣江模板被证明是展示

三倍内部量子效率,俯拾皆是量子局限史托克效应采用选择性外延区。InGaN种植在纳米

semi-polar led赣江模板也有效地提高内部量子效率31%。同样的原位过程也是非常有效

的减小缺陷密度由一个数量级,增加光子提取效率系数两个。

就地过程包括原位硅烷处理和高温增长过快。这两个过程只需要标准MOCVD工具和因此

低成本、适合批量生产。对待c-plane原位硅烷处理样品和硅烷在干邑氨环境,产生纳米

结构和截断锥200海里的规模。这些截锥结构可随后转化为金字塔的奈米结构组成,只有

10 - 11)和(11-22)semipolar飞机采用高强度增长过快。这些过程都被运用InGaN活跃

的地区和LED表面向内部量子效率的提高和光子提取效率,分别。广泛的材料、设备、光

学特征进行了这个研究。

1.1氮化镓材料为光电应用

氮化镓基材料,包括赣、氮、旅店、及其合金的优秀候选人是short-wavelength光电应

用。他们直接从紫外线,bandgaps延长近红外。此外,他们展示的力学性能和热稳定性高

比起其他iii - v族半导体、使他们特别适用于大功率和高温操作。近年来,突破方面减

少兴奋剂,缺陷导致了半导体激光器的商业化敢为基础,发光二极管(led)、高电子迁移晶

体管(HEMT)和氢探测器。尽管有这些进展,许多技术的挑战,例如绿色差距、基底增生仍然

存在。

许最重要的应用基础材料为天干solidstate照明(SSL)协议。在世界范围内,20%的电力

消耗照明构成,而其效率远低于25%。相比之下,空间加热效率已超过90%。为此,发展高效、

可靠的led固态照明,积极参与了行业和学术界都在过去的几年。计划由美国能源部,到

2015年,如果成功的话,solidstate照明可以减少整体电力消耗25%的能量。

与砷化镓半导体并输入为基础、赣的基础材料,遭受高密度的缺陷非常有限的可用性的

由于晶格匹配敢部份。到目前为止,大多数干邑光电子器件制造基础已经使用

hetero-epitaxy外国衬底,如蓝宝石(氧化铝),碳化硅(SiC),氮化铝(氮),在一个非常小

的百分比硅。因为大量的晶格失配、赣生长在这些基质经常展现一个高密度的线程的混乱

局面,通常以108 - 1010 /每平方厘米。这些缺陷的仍是主要限制因子干邑的性能基础光

电子器件,作为non-radiative重组、散射中心。实现较低的缺陷密度也会提高设备的可

靠性,从而延长寿命。减少各种缺陷的方法,如磊晶侧向over-growth(ELOG),并提出了一

些细节Chap.1.3.1讨论。如部份这篇论文中,我们探索了一种新的方法以有效降低使用奈

米结构干邑线程脱位密度。

在各种各样的磊晶技术,研制开发了具有一定特色的赣基材料、有机化学汽相淀积

(MOCVD)是国内领先的技术。典型的生长温度为赣江材料是在1000 - 1200°C。这种高生

长温度是必要的,以提高晶体质量和效果的效率低开裂氮源、氨(NH3),较低的温度。在第

二章中,我将总结我的贡献了MOCVD成功渐变工具的led磊晶成长干邑的研究。

1.2InGaN为固态照明led

SSL的基本组成是白光LED。如图1 - 1,它能够实现各种颜色的混合成分,这可以从可

产生直接输出或个人的led color-conversion材料,如磷。到目前为止,市面上买得到的

白光led通常包括一个蓝色的发射器和一个黄色的磷板。有证据表明,基于InGaN发光二

极体(led)外部量子效率能达到超过70%[1,2]。然而,这di-chromatic配置通常有一个可

怜的呈色指数由于缺少绿色和红色组件。启明星转换过程也限制了整体发光效率

downconversion由于能源中损失。特点:发光效率达到超过200镑/ W和颜色渲染指数

(CRI)超过90,这是普通照明需要,进一步提高效率和使用蓝色LED的tetra-chromatic结

构(蓝+绿色+黄+红色)是必要的[3]。*不幸的是,两InGaN效率明显下降,AlInGaP led在

green-yellow(500 - 580海里)范围。这个效率差距也被称为“绿色的差距”。因为

AlInGaP间接bandgaps材料在这个波长范围,实现高效率的SSL,这是至关重要的,明显改

善了明亮

注意到三色(如绿色+红+蓝)源无法取得CRI > 90。绿色和黄色InGaN效率的发光二极

管。在这篇论文中,我们将解决这些挑战干邑使用奈米结构。

图1 - 1。白光led说明各种潜在配置(在文献[4])。

1.3限制因素为InGaN led效率

到目前为止,效率InGaN led是由材料缺陷仍然有限,极化费用和光子捕获。在本部分,

我们将简要回顾和综述的状态如何theart这项研究帮助解决这些限制。

1.3.1材料缺陷

如上所述,高缺陷密度在赣基材料foreignsubstrates增加种植non-radiative重组率,

降低辐射效率。到目前为止,几个技术已经被证实对改善晶体质量,降低线程(TD)密度脱位

的赣层。pretreatmentat生长基质温度在合成氨环境,也称为氮[5],已被证明是高质量的

关键epilayers干邑。一个典型的密度在c-plane赣江层生长蓝宝石衬底可以减少到108

/平方厘米[8]的组合运用低温(LT;450 - 600°C)成核层(无限)和一个短退火的生长温度

变化的相位,无限as-grown从立方六角。[9 - 11]。作为第二章将会讨论,仔细优化这些

低温生长能明显地改变序列赣江模板增长。随后为了达到这个目标,一个国产光学原位监

测工具(反射器)的成立,将在第二章进行了广泛的交流。

此外低温缓冲增长,但是磊晶(ELOG)横向变化的选择性外延(SAE)地区介绍[12,13]进一

步降低其密度的一个数量级,低于107 /平方厘米。ELOG——包括pendeo的变化(从拉丁

语:挂在或暂停)7磊晶(PE)[14]和多级ELOG也是非常有效,以进一步降低TD密度。额外

的技术,如锡探讨多孔网络[15]和阳极氧化铝nano-mask[16]已经被提出和论证。所有这

些方法,然而,需要练习,因此会增加就地处理的复杂性和成本的制造。在这篇论文中,我们

将探索并推广了一种原位硅烷治疗方法有效地降低密度的一个数量级。

1.3.2极化电荷

由于具有对称的non-cubic赣江材料、compressively-strained活跃的地区,在InGaN

led具有自发和压电极化的指控。这些指控的内在诱发极化电场(IEF),通常以MV /厘米,

在活跃的地区,导致双方在高注入效率下垂电流密度和辐射效率的降低越来越发射波长。

可以分开的电子IEF孔和增加电子泄漏,导致内部量子效率低(IQE)和效率下垂[17],分

别。镇压IEF,预计这将增加IQE和电流密度在效率下垂发生,可以达到减少了晶格失配

hetero-structures他们在成长semi-polar(例如。{ 10 - 11 }和{ 11-22 })和无极(例

如a-plane和m-plane)表面。因为铟公司是更加困难的飞机上比在semi-polar无极的飞

机,这更有利于制造led semi-polar飞机在长波green-yellow抑制IEF。

至少有三个方法semi-polar InGaN制作led已报道的剧情。这些包括生长的基质在赣

epilayer尖晶石[18],体积上敢基质[19-27],壁上的金字塔或土坎敢结构建立在极性敢

表面平面使用SAE(28 - 35)。赣种植基质展出尖晶石迄今为止高密度位错和叠穿,从而降

低潜在的提高效率从降低IEF。使用散装semi-polar敢基质的优势,提出了降低IEF为了

提高效率的绿色和黄色发光二极管(25,26]。然而,如禁止地限制高成本和小晶圆片基片尺

寸需要解决这种方法可以变得更前实用。另一方面,技术可以创造semi-polar SAE的飞机

对极地敢表面。

高质量的极地敢薄膜,研究了制作多种底物包括蓝宝石,6 H-SiC,大部分由MOCVD干邑。

利用三维各向异性和增长速度增长,不同的semi-polar和无极敢飞机可生成c-plane敢

[13]。在第三章,我们将表明,高质量的InGaN多重量子井(MQWs)具有IQE大如三要素之一

极地MQWs相比,可以种植在金字塔敢微结构。然而这种方法需要例原位模式过程和不容易

产生一个平面结构,电触点。在这篇论文中,提出了一种新的研究semi-polar LED屏结构,

是一种新颖的纳米使得由外延,即纳米semi-polar天干(NSSP),可直接在赣c-plane制作,

但没有问题的上述方法SAE(36)。NSSP敢也消除了过量的缺陷问题为赣江生长在尖晶石载

体和降低了成本使用散装semi-polar敢部份。当我们后面将会说明,表面NSSP敢包括两

种不同semi-polar飞机(10 - 11)和(11-22)。因此,预计InGaN活跃的地区在NSSP制作

赣能表现出低IEF,因此大大提高IQE。

1.3.3光子提取

光子被后产生的活跃的地区在led,他们需要摆脱设备为了是有用的。当光从一个更高

的折射率介质中折射率较低,总内部反射(TIR)发生在界面。在InGaN LED、光子在经历了

TIR表面可以re-absorbed活跃的地区或被困在装置由于wave-guiding效果如图1 - 2

所示。在一个简单的InGaN主导,只有4%的活跃的地区产生的光子能逃避每台设备表面。

有证据表明,在表面纹理的表面可以大大降低TIR领导,提高光子提取效率如图1 - 2。到

目前为止,许多表面贴图技术,如光子晶体结构[37]和photo-electrochemical干邑的蚀

刻画[38]表面作了介绍。值得注意的是,photo-electrochemical nitrogen-terminated

敢表面蚀刻,已成功应用到商业发光二极体(led)[2]。然而,这些方法都需要额外的例原位

模式过程大大增加成本。在这篇论文中,我们研究一个原位过程制造奈米结构

赣表面led,有效改善光子提取效率。图1 - 2。在光波导旅行(左)与一个光滑界面和(右)

与粗糙界面([39])。

1.4组织论文的

摘要本论文是探讨奈米制造技术,成本可显著提高效率的最先进的InGaN led在两个蓝

色和绿色/黄色高性能固态照明范围。这篇论文的组织是这样的。

第二章总结了MOCVD技术给出了InGaN发光二极管。在第三章,我们研究了在InGaN依

赖IQE { 10 - 11 } semi-polar飞机采用SAE。在第四章里,制备、表征及小说和精打细

算的奈米结构模板将被描述。干邑摘要用原位硅烷处理(ISST)和高温罪恶(HTO),形成纳米

倒锥结构和奈米结构semi-polar模板(NSSP)取得了。在第五章,我们研究的基础上InGaN

semi-polar led NSSP模板。一个完善内部量子效率进行了论证。绿色semi-polar InGaN

生长在一个c-plane衬底LED也进行了论证。在第六章中,电流分散在NSSP InGaN led,

将会被讨论。第七章,应用ISST的提高光子提取效率的InGaN LED,将会被讨论。在第八

章,我们将总结并提出一些建议供未来的工作。

2.1氮化镓成长

正如前面提到的介绍,氮化镓(赣)及相关合金未来固态照明的优秀候选人。到目前为

止,III-nitride磊晶成长已经被局限缺乏足够大homoepitaxial单晶基质的增长。因此,

生长相关的材料已经赣江和这在很大程度上是基于使用氢化物原子hetero-epitaxy汽相

(HVPE)、金属有机化学气相沉积(MOCVD),分子束外延(勋章)。在这些技术、MOCVD技术领

先是由于原材料的质量优势,可扩展性,而成本(40)。材质的MOCVD干邑都是完美增长,由

于其相对高的生长温度(1000 - 1200°C)【41,42]。

到目前为止,各种基板材料包括蓝宝石(氧化铝),碳化硅(SiC),研究了硅对赣江增长(见

表1 - 2)。虽然敢基质最近通过引入市场散装材料生长基质,用HVPE国外激光切割沿特

定的水晶飞机,成本已经禁止地高。另一方面,敢上生长蓝宝石衬底(0001)c-plane展品稳

定增长在大范围的发育条件尽管脱位密度高基体之间的界面磊晶层。在这项研究中,我已

帮助了一个加快MOCVD系统与博士申雪俞思远。在这一章中,我将总结MOCVD技术策略和

缺陷减少InGaN发光二极管(led)外延,将用于整个论文。

2.1.1使用MOCVD赣江增长

由于庞大的晶格失配赣江和蓝宝石之间,是一个重要的含缺陷界面附近/蓝宝石干邑,缺

陷密度可以最小化器件地区。达到最优化的用原位反射器(44,45]。一个自制的反射器系

统是建立在图2 - 1在我们3×2”Thomas-Swan座小的喷头以便节省热水(CCS)的MOCVD

系统。白色的光反射表面和监测样品一个光谱在增长。敏感的反射率无论表面形态和外延

层结构。

图2 - 1。说明一个自制的原位反射器系统集成到MOCVD系统。

图2 - 2。典型的生长条件,应用于该研究赣江模板。

典型的生长条件,应用于该研究赣江模板进行了总结和表2 - 2在图2 - 2。除非另有

提到,c-plane蓝宝石基板的方法。在五个步骤表2 - 2,包括高温(HT)清洗、氮、低温度

(LT)成核、退火的成核层肝移植,HT敢增长,是至关重要的epilayer高品质干邑。

图2 - 3和表2 - 3显示相应的原位反射器的信号。在下面,我们将描述如何反射器的

信号可以用来优化敢模板的增长。除非另有提到,我们指的是一种反射器的信号显示在图

2 - 3。

图2 - 3。原位反射器的痕迹模板增长天干(抽样身份证:UM-S07 - 254)。对应的突出

领域中重要的sub-steps外延。

2.1.1.1高温清洗

最初,作为样品温度大幅增加了,由于反射率增加增加的折射率的样品。金等问题的影响

已经彻底地研究初始热清洁蓝宝石衬底和实验,证明这种热处理可以有效地降低表面粗糙

度的基质(46)。一般来说,平面最好敢核形成统一,发展的关键,是水晶的最后epilayer

干邑质量。具体条件应优化HT清洗通过检查处理温度和时间。在我们敢生长,最佳处理温

度和时间设置为1075°C和5分钟,分别。此外,高温退火可有效消除表面表面水分。

2.1.1.2氮

氮(5、7]是一个过程,NH3 preflow氢(H2)环境下准备的表面的成长。在氮、NH3起反应

与表面氧原子的蓝宝石衬底材料。由于更换氧原子被氮原子的扩散和氮原子成一定的深

度、表面光滑暴露成为非晶态。因为这表面形貌的变化以数以埃米,相应的反射率变化不

显著。结果表明通过适当的渗氮条件下,低密度epilayers干邑脱位和更好的电气和光学

性能可达到[7]。然而,如上所述,合适的组合反应器的条件如温度、处理时间、和氨流量

必须加以考虑。在我们敢生长,在530°氮进行优化C,总数为210秒应低于三大量NH3流

动。

2.1.1.3低温核

作为1.3.1节所述,几种方法被引入减少螺纹(TD)密度错位发展敢模板。具体地说,用低

温核层(LT无限)已被证明是简单而有效的。一个线程脱位低密度为108 /平方厘米被报道

[8]。

作为nucleated干邑上是蓝宝石、立方相位群岛是第一次成立在温度为450 - 600°C。

这些岛屿wurtzite随后转化阶段[8]。增加的反射率在肝移植是由于无限增长的增加反映

平板表面的细胞核。基本上,我们知道反射是关于两次从天干强于蓝宝石由于折射指数的

差异。当岛屿变得致密(即成长时间的无限成为肝移植的时间,从顶面全反射细胞核变成

200%的反映蓝宝石衬底的假设整个表面覆盖着赣江群岛。虽然群岛是完全不合并形成一个

水晶层,这仍然是有可能的,因为邻近岛屿之间的距离太小比光的波长。一旦反射超过两倍

的蓝宝石(如图2 - 3),海岛继续进一步融合,导致谷物和一件厚的较大的干邑无限。在这

里,成核的大小岛屿和厚度的无限可获得高质量的关键epilayer干邑。表明,我们已经把

一系列敢模板与不同的无限的条件。在保持一致的所有条件都得来的成长时间以外的肝移

植是无限多样,导致不同的LT无限厚度。肝移植的厚度是通过分析无限推算反射比数据为

反射器末无限增长肝蓝宝石衬底(RLT无限/ RSapphire)。品质的赣模板进行表征发光

(PL),用x射线衍射仪(XRD)。从以上结果,最好的赣模板质量/ RSapphire无限时取得RLT

约2.6与一个40海里厚无限,在给予生长条件。

LT无限生长温度得来= 530°C,V / 3 = 9140、轻卡无限退火时间= 420秒,HT干邑=

1040°C的生长温度,V / 3 = 1230、生长时间= 4300秒。

图2 - 4。赣模板的比较就反射品质之比表面和蓝宝石LT无限板上。

2.1.1.4退火温度低成核层

在赣hetero-epitaxy和一个巨大的晶格失配、初始生长遵循Volmer韦伯表面模型[47],

即增长。敢岛居多”。为了获得平滑敢模板,这些岛屿需要逐层转化使用一个无限增长模

式退火工艺。在退火过程中,逐渐增加基片温度达1030 - 1050°C下高压氨。温度增加速

率、反应压力、NH3流可以控制国联分解率,决定了表面粗糙度末端的退火工艺[48,49]。

图3、4点后,肝移植(h)无限退火开始,通常轻微上升反射率观测到。增加大约800°C持

续直到在赣分解过程开始发生。一旦反射强度的山峰,它开始下降由于增加表面粗糙度。

最初随机分布的岛屿开始被转化成较均匀的岛屿由于国联的分解和迁移的ad-atoms镓。

在退火过程中,先降低由于反射率增加表面粗糙度。结果进一步退火的反射率有轻微的

上升,因为在更高的温度、表面形貌变得平滑。然而,如果我们进一步退火表面,表面粗糙

度再次增长,导致的反射强度降低[48,49]。这种现象可以解释为天干考虑体积的岛屿。摘

要转型点((k)在图2 - 3),体积的单位面积上的岛屿,成为最高优先为后续HT敢增长。就

想拇指规则一样,它的位置(反射器微量)肩主导下的退火工艺最高温度(约50]。总之,目

标在低温的形核密度以及随后的退火是完成一个表面形貌与适当的密度和尺寸的岛屿以

下HT敢增长。

如图2 - 5所示,甚至一个分布的微小变化引起的岛上的细微差别国联厚度和温度增加

率(表2 - 4)会导致显著差异,在接下来的HT干邑相同条件下生长。一般来说,要花更多

的时间与一个无限的粗糙表面和较小的岛屿被转化成二维的增长模式。条件,达到了较高

的水晶质量上主要涉及干邑蓝宝石是成长与退火的LT无限。

2.1.1.5 HT增长

一旦蓝宝石表面覆盖着合适的体积、均匀性、厚度和密度的岛屿天干、HT敢增长可以

遵循。这HT敢本身可分为两部分(图2 - 6)。第一部分相当于初期生长在赣HT转变增长

模式从3 D,2 D,影响水晶质量有明显提高。在第二部分、赣epilayer变厚,因为增长方

式以及稳定增长的条件下对二维模式。几个策略来控制敢增长将简要地介绍了每一个政权

在下面。

增长的部分,我是一个缓冲一步准备一个表面适合HT敢增长。在这一步中,反射器的震

荡信号变得越来越明显。最初,反射率持续降低由于表面粗糙度的增加使岛屿的江水,即。

3 D的增长。随着时间的推移,3 D抑制增长模式以及二维增长模式增强。一旦表面变得扁

平由于提高二维增长,一层层地开始成长的干邑,导致其反射率增加。这一部分的时间增长

可以藉由调整优化反应器压力,V 2/ III比,增长率[51,52]。例如,在一个较低的情况

下,V / III比,需要更长的时间去恢复反射强度,这意味着增长方式的变化(3 D D)发生的

更慢。的反射率的恢复时间是关键振荡幅值在此部份填报。一般来说,一个更大的振动幅

值对应一个更好的晶体质量。

第二部分的高温稳定赣江增长广泛生长发育条件,因为发生质有限的地区。不过,几个关

键因素还会影响到水晶结构,包括生长温度,trimethyl-gallium(TMG)流量、NH3流动,V /

III比、反应堆压力。如图2胜7负,其增长率随III族流增加而降低的V / III比和生

长温度增加。增长速度是一个重要的参数来确定光学和电学性能,特别是对赣江epilayer

p - n -型或掺杂的病例。这将是更详细地讨论的在下一个部分。

第二篇：

[ 所译外文资料：

①作者：Primary Examiner:Lobo, Ian J.

②书名（或论文题目）：Ultrasonic distance meter

③出版社（或刊物名称或可获得地址）：Document Type and Number:United States

Patent 5442592

④出版时间（或卷期号）：08/15/1995

⑤所译起止页码：]

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