eLearning

X Z Shang1,3
,SDWu2
, C Liu1
,WXWang2
, LWGuo2
, Q Huang2
and J M Zhou2
1
School of Physics and Technology, Wuhan University, Wuhan 430072, Hubei, People’s
Republic of China
2
State Key Laboratory for Surface Physics, Institute of Physics, Chinese Academy of
Sciences, Beijing 100080, People’s Republic of China
E-mail: xunzhong@acc-lab.whu.edu.cn
Received 8 December 2005
Published 20 April 2006
Online at stacks.iop.org/JPhysD/39/1800
Abstract
Low-temperature step-graded InAlAs metamorphic buffer layers on GaAs
substrate grown by molecular beam epitaxy were investigated. The strain
relaxation and the composition of the top InAlAs layer were determined by
high-resolution triple-axis x-ray diffraction measurements, which show that
the top InAlAs layer is nearly fully relaxed. Surface morphology was
observed by reflection high-energy electron diffraction pattern and atomic
force microscopy. Under a selected range of growth parameters, the root
mean square surface roughness of the sample grown at 380 ◦
C is 0.802 nm,
which has the smallest value compared with those of other
samples.Furthmore, The ω-2θ and ω scans of the triple-axis x-ray
diffraction,and photoluminescence show the sample grown at 380 ◦
C has
better crystalline quality. With decreasing As overpressure at this growth
temperature, crystalline quality became poor and could not maintain two
dimensional growth with increasing overpressure. The carrier concentrations
and Hall mobilities of the InAlAs/ InGaAs/GaAs MM-HEMT structure on
low-temperature step-graded InAlAs metamorphic buffer layers grown in
optimized conditions are high enough to make devices.

1. Introduction
Compositionally step-graded metamorphic layers, changing
the composition of epilayers along the direction of epilayer
growth step-by-step, have received considerable attention as
buffer layers for large lattice mismatch epilayers on GaAs
substrate [1]. This buffer can accommodate the large lattice
mismatch between the active layer and the GaAs substrate
by the formation of misfit dislocations, isolate these misfit
dislocations and prevent their propagation into the active layer
grown on the buffer [2–4]. It is widely used as buffer layer for
microelectronic devices, such as metamorphic high electron
mobility transistor (MM-HEMT) [5] and heterojunction
bipolar transistors (HBT) [6]. Various metamorphic buffer
3 Author to who any correspondence should be addressed.
layers have been grown onGaAs substrate, such as InGaAs [7],
InAlAs [8], InGaP [9], AlGaInAs [10] or AlGaAsSb [5].
Among these materials, InAlAs metamorphic buffer layers
have many advantages, for example, Al containing buffer has
less leakage current than that of InGaAs, i.e. the presence of
Al improves the insulating properties [3]. However, owing
to the presence of Al content, the crystalline and optical
properties of the InAlAs layer is found to be rather poor.
Furthermore, clustering effect seems to occur, due to the large
difference between the In–As and Al–As bond energies [11].
The amount of clustering is expected to be strongly dependent
on growth conditions (growth temperature, V/III ratio,etc)
particularly the growth temperature. The metamorphic step-
graded buffer layers were usually grown at low temperature
(LT) (Ts
400 ◦
C) to avoid island formation, confine themajority of dislocations
to within the buffer layers, prevent
their propagation into upper layers [12, 13], significantly
suppress buffer leakage current and miscibility gap clustering
in InAlAs [14]. LT growth method can change the strain
relaxation dramatically and incorporate a very high density
of defects [15], furthermore, device applications often require
high resistivity, low lifetime material to act as buffers for
electrical isolation and reducation of backgating and sidegating
[16]. The step-graded buffer layer filter dislocations more
easily as growth proceeds than the linearly graded InAlAs
buffer layer [17]. In xAl1−x As epilayers with the In
composition of x=0.52 are mainly used as barrier layers due
to their large band gap (1.45 eV at 300K). In0.52Al0.48 As
(hereafter written as InAlAs) whose lattice matches the InP
substrate and In0.53Ga0.47 As(hereafter written as InGaAs)
active layer for long wavelength communication systems [18].
Some groups researched the InAlAs epilayer grown on InP
at a low temperature [11, 18], but few reports were focused
on low temperature step-graded InAlAs/GaAs metamorphic
buffer grown by molecular beam epitaxy(MBE).
In this paper, we study the characterization of step-graded
InAlAs buffer on the GaAs substrate under a selected range
of growth parameters. The influence of growth parameters is
studied by high-resolution x-ray diffraction(HRXRD), atomic
force microscopy(AFM) and photoluminescence(PL).The
electrical properties of the InAlAs/ InGaAs/GaAsMM-HEMT
structure grown on low-temperature step-graded InAlAs
metamorphic buffer layers are reported.