Effects of Si-doping on the Microstructure of AlGaN/GaN Multiple-quantum-well

R. Liu and F. A. Ponce

Department of Physics and Astronomy, Arizona State University, Tempe, AZ 85287

S-L. Sahonta and D. Cherns

H. H. Wills Physics Laboratory, University of Bristol, Bristol BS8 1TL, UK

H. Amano and I. Akasaki

Department of Materials Science and Engineering, Meijo University, Nagoya 468, Japan

ABSTRACT

The effects of silicon-doping on the microstructure of Al

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N/GaN multiple-

quantum-well (MQW) have been studied by TEM. Significant changes of surface

morphology and dislocation core structures have been observed due to Si-doping in the

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N barriers. Threading dislocations create surface pits in the MQW as a result

of Si doping. With an increasing doping level, the pits change the shape from small

faceted pyramid to large cone. The formation mechanism of the surface pits has been

discussed from both dynamics and kinetics points of view. We have also observed

nanopipes constrict to form closed core screw dislocations in the MQW due to Si-doping.

INTRODUCTION

Applications of UV-LEDs using GaN active regions have been obstructed by the

weak luminescence efficiency [1]. In comparison to InGaN quantum well (QW), GaN

QW appears to suffer more from the non-radiative recombination nature of threading

dislocations and the quantum-confined Stark effect caused by the internal field [1]. This

is simply because GaN QW is free of alloy fluctuation, which is believed to be essential

for a high radiative recombination rate in InGaN QW [1]. For further improvement of the

luminescence efficiency, Si doping in barrier layers is promising, because the doping

design is expected to improve radiative efficiency by screening the internal field that

separates the electron-hole wave function [2]. Si doping in InGaN barrier layers in

InGaN/InGaN MQW has been found to improve the optical gain of the laser diodes [3].

A study of the optical properties of Si-doped AlGaN/GaN MQW has been reported

earlier [2]. In the present work, the doping effect on the microstructure is studied by

TEM.

EXPERIMENTAL

The studied samples are a set of three Al

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N/GaN MQW structures grown on

GaN by metal-organic vapor phase epitaxy. Detailed growth parameters were reported

earlier [4]. The MQW consists of 5 periods of AlGaN/GaN structure. The difference

between the samples is the Si doping level in the AlGaN barriers. The first sample is

nominally undoped. And the other two have very high doping levels, which are reflected

by their high carrier concentrations: about 4x10

-3

and 9x10

-3

respectively. In

comparison, the background carrier concentration for the nominally undoped one is about

1x10

-3

. The microstructure of these samples has been characterized by TEM in

cross-section and plan-view. The microscopes used in this study are JEOL 4000FX

operated at 400kV and Philip CM200 FEG operated at 200kV. The foil specimens were

prepared by mechanical polishing followed by Ar

ion milling.

RESULTS AND DISCUSSION

Surface pits at the ends of threading dislocations

One significant effect of Si-doping on the microstructure is inducing surface openings

at the ends of threading dislocations. Figure 1 shows TEM images of the undoped

sample taken in cross-section and plan-view. In Fig. 1(a), a mixed type threading

dislocation propagates from the underlying GaN layer into the MQW structure and does

not create a visible opening on the surface. Similarly, Fig. 1(b) shows a nanopipe

propagates through the MQW structure without changing its shape. Figure 1(c) shows a

plan-view image of this sample. There is no evidence of surface pits at the ends of the

threading dislocations. However, the microstructure is significantly changed when the

AlGaN barriers are doped with Si. Figure 2 shows a set of TEM images of the second

sample with a Si-doping level about 4x10

-3

. In Fig. 2(a), a cross-section image

shows many surface pits. Close examination indicates that they in fact originate from

threading dislocations. Since the image was taken under g=0002 diffraction condition

g=0002

nanopipe

g=11-20

nanopipe

30nm

Figure 1. Bright field TEM images of the

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N/GaN MQW on GaN without Si-

doping in the barriers: (a) and (b) are taken in

cross-section, and (c) in plan-view. Threading

dislocations form no pits on the film surface.

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to show threading dislocations with a screw component in strong contrast, those of pure

edge nature only show faintly (ideally should be invisible). In Fig. 2 (b), we observe a

nanopipe propagating from the GaN template changes to a full dislocation and form

opening on the surface. In short, it appears that all the threading dislocations, regardless

of the Burgers vector, form surface pits due to Si-doping. Morphology of the surface pits

is quite complex. In cross-section, they look like tulips; while plan-view imaging shows

that they are bound with six-fold symmetry facets. The actual morphology probably

consists of two parts: one is the faceted pyramid bottom part and the other is the cone-

shape top. However, in the higher doped sample, the surface pits are completely cone-

shape. In addition, we also notice that threading dislocations start the formation of the

surface pits earlier with a higher doping level. The surface pits formed almost

immediately when the deposition of the MQW started (Fig. 3(a)). The differences

between the two samples may give some clues when we analyze the pit formation

mechanism.

Pit formation mechanism

In InGaN/GaN MQW structures, similar surface pits have been observed by several

groups [5-6]. The pits were found to be bound with six {101

1} facets. There were some

arguments about the formation mechanism of these defects [5-8]. Relaxation of lattice

g=0002

g=0111

nanopipe

g=0002

nanopipe

Figure 2. Bright field TEM images of the Al

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N:Si/GaN MQW on GaN with a

doping level about 4x10

-3

: (a) and (b) are cross-section images, and (c) is plan-view.

Threading dislocations form surface pits with six-fold symmetry. Nanopipes change to

full screw dislocations and form surface pits in the MQW.

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mismatch strain was believed to be the driving force of the surface pit formation [5,7].

However, it was argued that a reduced incorporation of Ga atoms on the {101

1} facets

results in the surface pits [6]. First-principles calculations suggested that indium

segregation on the facets may reduce the surface free energy and thereby allow formation

of the relatively large size surface pits [8].

In the present work of Al

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N/GaN MQW, we found no surface pits for the

undoped sample. When the barriers were doped with Si, threading dislocations formed

V-shape pits on the surface. In the sample with the higher doping level, the pits are much

larger and no specific facets are formed. This observation suggests that Si impurities

probably reduce the surface free energy, which is similar to indium in the study of

InGaN/GaN MQW [8]. There are two possible ways to explain the observations. One is

via analyzing the doping effect on the system energy (from a dynamics point of view),

and the other is to consider the doping effect on the growth kinetics (from a kinetics point

of view). Following Frank’s theory for determining equilibrium radii of hollow core

dislocations [9], it has been proposed that the pit formation energy consists of three terms:

(1) the change in surface energy resulting from replacing a hexagonal region of the (0001)

surface with the six {101

0} facets of the inverted pyramid pit, (2) the reduction in strain

energy (around threading dislocation cores), and (3) the elimination of the dislocation

core energy [8]. In addition, there may be another term need to be considered: (4) the

reduction in lattice mismatch strain energy, which results from the surface roughing

caused by pitting. The reason why the undoped sample do not exhibit surface pits might

be that the critical thickness for the pit formation is not exceeded, i.e. the first term is

larger than the sum of the other three. This speculation can be justified by comparing

lattice mismatches between AlN/GaN and InN/GaN. For AlN/GaN, the value is about -

2.4%, which is less than one quarter of the magnitude between InN and GaN (about 11%).

Thus, AlGaN alloys grown on GaN will have much reduced lattice mismatch strain

compared with similar composition InGaN alloys. In addition, Al content is only 7% in

this study. Therefore, the critical thickness for the surface pit formation is probably

larger in the currently studied Al

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N/GaN MQW structure. When Si impurities

were introduced into the growth, the surface free energy might be reduced. This

decreased the threshold energy for the pit formation (the first term) to such a point that it

was even lower than the sum of the other three terms. In consequence, surface pits were

created at threading dislocation ends to minimize the system energy. However, the

validity of this explanation remains to be confirmed because Frank’s theory has been

found to be significant only for core radii less than 1nm in GaN [9]. Therefore, it is

sensible to consider an alternative explanation from a kinetics point of view, i.e. the pit

formation solely results from Si-doping effect on growth kinetics. On the growth surface,

threading dislocations generate localized strain field in the vicinity of their cores, which

may be attractive for impurity segregation. If Si impurities segregate at the ends of

threading dislocations, they may retard the growth of the dislocation core. As a result,

facets like {101

1} can be generated and open up in the following growth because Si

impurities may help to stabilize the facets. We can also explain the delay of the pit

formation in the sample with the lower doping level by the need to build up the surface Si

concentration to a point where stable facets can be formed. In short, further study is

needed in order to determine which factor is playing the key role in the pit formation.

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Closure of nanopipes

In Fig. 2(b-c), we show that nanopipes constrict to form closed core screw

dislocations when they propagate into the MQW structure. Doping of Si appears to be

responsible for the transformation, since the same defects in the undoped sample

maintain the coreless feature (Fig. 1(b-c)). In the sample with the highest doping level,

nanopipes form surface pits before they have a chance to become full dislocations (Fig.

3(b)). The origin of the core structure transformation is puzzling. It cannot be accounted

for by Frank’s model [10], because reduction in surface free energy due to Si doping

should increase the core radius rather than decrease it, provided that the strain energy

around the dislocation core remains the same. However, Si doping has been reported to

increase the surface roughness by enhancing spiral growth mode [11,12]. It is probably

an accelerated spiral growth of the nanopipe that leads to the constriction of the opening

cores.

SUMMARY

The effects of Si-doping on the microstructure of the Al

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N/GaN MQW

structures have been studied. According to our observations, all threading dislocations,

regardless of the Burgers vector, generate surface pits in Si-doped AlGaN layers. With

an increasing doping level, the originally pyramid-faceted pits become cone-shape. The

formation mechanism of the surface pits due to Si-doping has been discussed from both

dynamics and kinetics points of view. In addition, Si-doping also results in closure of

nanopipes in the MQW. The effect of Si-doping on growth kinetics is probably

responsible for the transformation. This observation may provide a new way to get rid of

the detrimental effects of nanopipes on device performance.

g=0002

=11-20

nanopipe

Figure 3. Bright field TEM images of the

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N:Si/GaN MQW on GaN with a

doping level about 9x10

-3

: (a) and (b)

are cross-section and plan-view

respectively. Threading dislocations form

deep surface pits in circular funnel shape.

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ACKNOWLEDGEMENTS

This work is supported by an international NEDO (New Energy and Industrial

Technology Development Organization) program (NO 01-MB10).

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