This dissertation covers the study of the surface properties and the crystal quality of Ge epitaxial film grown on InGaP/GaAs substrates. In chapter 2, the characterization of the In 1-xGaxP/GaAs system according to the literature is introduced. In chapter 3, the UHVCVD system and experimental procedure in the study are demonstrated. Then, various analytic equipment to characterize the material quality and device performance of Ge/InGaP/GaAs structures would be presented.
In chapter 4, the experiment results are shown and discussed. High quality epitaxial Ge were grown on InGaP/GaAs substrates by ultra-high vacuum chemical vapor deposition (UHVCVD). The discussion about the incubation time and growth rate, growth mode, surface morphology, crystal quality, and interdiffusion will be presented here. Finally, the conclusions will be in the chapter 5.
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Table 1.1 Material properties of popular semiconductors
Composition x Material
Lattice Constant (Å )
Thermal Expansion Coefficient (10⁻⁶/K)
Energy Band Gap (eV)
x= 0 InP 5.869 4.60 1.34
x= 0.5 In0.5Ga0.5P 5.653 5.35 1.76
x= 1 GaP 5.451 4.65 2.26
Ge 5.658 5.90 0.66
GaAs 5.653 5.73 1.43
Table 1.2 Material properties of In1-xGaxP, Ge, and GaAs at 300 K
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Figure 1.1 The comparisons of lattice constant and energy band gap between semiconductor materials
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Figure 1.2 The prospects of p-channel MOSFET can be integrated with n-channel III-V material devices on the same GaAs template for complimentary architecture for beyond-the-CMOS-roadmap logic applications.
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Chapter 2
Literature Review: Characterization of the In
xGa
1-xP – GaAs System
2.1 III-V Ternary alloys [6]
When more than one element from group III or group V is distributed randomly on group III or group V lattices sites, III-III-V or III-V-V ternary alloys can be achieved. The notation most frequently used is IIIxIII1-xV or IIIVyV1-y. There are 18 possible ternary system among the group III and group V elements of interest.
The bandgap energy Eg(x) of a ternary compound varies with the composition x as follows:
Eg(x) = Eg(0) +bx +cx2 (2-1)
where Eg(0) is the bandgap energy of the lower binary compound and c is the bowing parameter. The bowing parameter c can be theoretically determined (Van Vechten and Bergstresser 1970). It is especially helpful to estimate c when experimental data are
unavailable. The lattice constant of ternary compounds can be calculated using Vegard’s law.
According to Vegard’s law the lattice constant of the ternary alloys can be expressed as follows:
Aalloy = xaA + (1-x)aB (2-2)
where aA and aB are the lattice constant of the binary alloys A and B. Vegard’s law is obeyed quite well in the most of the III-V ternary alloys. The compositional dependence of the energy gaps of carious III-V ternary alloys at 300K is given in Table 2.1 (Casey and Panish 1978).
7 2.2 InxGa1-xP/GaAs system [6]
2.2.1 Introduction
In the past few years, AlxGa1-xAs /GaAs heterostructure have emerged as a promising system for optoelectronics and microwave device applications. However, because of the strong reaction between Al and oxygen, even trace quantities of oxygen have a dramatic effect on the quality of AlxGa1-xAs layers due to the effective introduction of deep-level defects. One of the solutions is to replace AlxGa1-xAs by InxGa1-xP/GaAs.
The electrical, optical and structural properties of InxGa1-xP/GaAs depend directly on how the system is lattice matched. Concerning ΔEc, there is a surprise: if we assume that the discontinuity in the conduction band is the difference in the electron affinities (χ) of χ(GaAs)
= 4.05eV, χ(InP) = 4.4eV, χ(GaP) = 4.0eV (the electron affinity of InxGa1-xP is take as the average of χ(InP) and χ(GaP) and χ(In0.49Ga0.51P) = 4.2eV), then ΔEc = χ(GaAs) – χ(InGaP) = -0.15eV. However, the experiment results show that ΔEc = 0.2eV and ΔEv = 0.28eV.
The InxGa1-xP ternary alloy lattice matched to GaAs substrate has attracted a lot of attention not only because it is a good alternative to AlxGa1-xAs /GaAs -based devices but also because it is used as a model material to study the ordering effect and its influence on InxGa 1-xP properties.
2.2.2 Growth Details
InxGa1-xP layers can be grown by MOCVD, either at atmospheric pressure or low pressure and at low temperatures between 500 and 6000C. One can use different group III alkyls for Ga and In sources, and hydrides or alkyls for group-V P sources. Chemical reactions occurring among these sources are as follows:
0.51R3Ga + 0.49R’3In + EH3
H2→ Ga0.51In0.49P +nCnH2n (2-3) where R, R’ and E can be methyl, ethyl, alkyl or hydride.
The InxGa1-xP layers can be grown at low temperature, between 500 and 5500C, by using
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triethylgallium (TEGa), trimethylindium (TMIn) and pure phosphine (PH3) in H2 carrier gas.
The optimum growth conditions are given in Table 2.2.
The growth rate (dx/dt) of InxGa1-xP depends on the flow rates of TMIn and TEGa (group-III element) and is independent of PH3 flow rate (group-V element) and growth temperature under the growth conditions listed in Table 2.2. The distribution coefficients of indium and gallium are defined as
K= 𝑋𝐺𝑎𝑆 / 𝑋𝐺𝑎𝑉 (2-4) and
K= 𝑋𝐺𝑎𝑆 / 𝑋𝐺𝑎𝑉 (2-5)
are nearly equal to unity. Figure 2.1 shows the variation of growth rate dx/dt of InxGa1-xP lattice matched to GaAs with a growth temperature of TG= 5400C and growth pressure of 76 Torr. Similar results have been reported by Hsu et al (1985) at growth temperatures from 600 up to 6500C. They showed that there was no gas-phase reaction in their reactor leading to premature depletion of In or Ga.
An undoped InxGa1-xP layer grown under the conditions of table 2.2 has a free electron carrier concentration of 5 x1014 cm-3 with mobility of 6000 cm2V-1s-1 at 300K and 40 000 cm2V-1s-1 at 77K. No GaAs buffer layer is grown in this case (Razeghi et al 1989b).
9 2.2.3. Microstructure properties [7]
Compositionally abrupt InxGa1-xP /GaAs heterojunctions have been investigated by cross-sectional scanning tunneling microscopy (STS) and spectroscopy. The advantage of such work is that band offsets can be measured while simultaneously imaging the atomic-scale structural properties of the interfaces. Images inside the InGaP layer reveal a random arrangement of In and Ga atom. This result is consistent with PL results and growth conditions for similar samples that indicate a nearly fully disordered InGaP layer [8]. It is found that GaAs-on-InGaP interface has a slightly wider transition region and more interface intermixing than the InGaP-on-GaAs interface. Both interfaces exhibit InGaAs-like
properties. Indium outdiffusion from InGaP into GaAs at the GaAs-on InGaP interface is clearly identified, although As/P interchange is not very obvious. Spatially resolved spectra reveal that nearly all of the band gap discontinuity occurs between the valence band edges.
In the large-scale STM image of Figure 2.1(a) the InGaP layer is seen in the center part of the image with GaAs layers seen on either side. Growth direction is from the right to the left for all images presented in this paper. The InGaP layer appears mottled due to
compositional fluctuations in the alloy. A high-resolution image of the InGaP layer is shown in Figure 2.1(b). At a sample bias of –2.0 V, filled states are imaged, i.e. localized on P atoms for InGaP. The pattern of different brightness for the P atoms reflects the distribution of neighboring In and Ga atoms. Because the cleaved surface is atomically flat, the observed contrast arises from a combination of electronic and strain effects, both associated with the presence in the alloy of clusters that are InP-rich or GaP-rich [9]. Ordering of the alloy is an important phenomenon for InGaP.
High-resolution images of inverted and normal InGaP/GaAs interfaces were shown in the Figure 2.2 (a) and (b), respectively. Atoms on the group V sublattice are imaged here, i.e.
revealing As atoms in GaAs and P atoms in InGaP. Arrows indicate the nominal position of the interfaces. The two interfaces display different features. For the GaAs-on-InGaP interface,
10
the transition region is about 3-4 atomic bilayers (bilayer spacing is 5.65Å ) and most of it lies in the GaAs side. Some atom-size bright features are seen in the GaAs layer near the
interface. We attribute these features to In atoms. The image of In atoms will appear brighter because In atoms are bigger than Ga atoms (also the band gap of InAs is smaller than that of GaAs, which would contribute to a larger tunnel current near In atoms [9])
2.3. Single crystal Ge film on InxGa1-xP [10]
A 280 Å organometallic vapor phase epitaxy (OMVPE) InxGa1-xP film grown on (100) GaAs. The film was slightly gallium rich so that its x-ray rocking curve could be discerned from the GaAs and the Ge, which has a lattice parameter slightly larger than that of GaAs.
After that, 600Å Ge film was deposited on it at a rate of 0.3-0.5 Å /s.
The (400) GaAs rocking curve can be seen in Figure 2.3 for the GaAs/InxGa1-xP/Ge structure along with the (400) InxGa1-xP and germanium rocking curves again suggesting that the films grew epitaxially. This is verified in the TEM micro-graph in Figure 2.4 which also illustrates that both InxGa1-xP interfaces are relatively smooth. The germanium film grew epitaxially with a smooth, abrupt interface.
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Table 2.1 Compositional dependence of the energy gap in the III-V ternary solid solution at 300 K
Table 2.2 Optimum growth parameters for GaAs and InxGa1-xP
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Figure 2.1 (a) STM image of p-GaAs/i-InGaP/p-GaAs heterostructure, acquired with sample voltage of 2.5 V. (b) High-resolution image of InGaP layer, acquired with sample voltage of – 2.0 V and displayed with a gray scale of 0.5Å . Growth direction is from right to left.
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Figure 2.2 STM images of (a) GaAs-on-InGaP and (b) InGaP-on-GaAs interface. Both image were acquired with sample voltage of –2.0 V and are displayed with gray scales of 0.9 Å . Growth direction is from right to left.
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Figure 2.3 Experimental rocking curves of 600Å Ge layer on InxGa1-xP /GaAs
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Figure 2.4 Ge film deposited on InxGa1-xP/GaAs (100) from the sample as shown in Figure 2.3 Epitaxial Ge with a smooth InGaP interface is observed in lattice fringes.
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Chapter 3
Ultra High Vacuum Chemical Vapor Deposition (UHVCVD) System, Experimental Procedure and Characterization Methods
In the chapter, the UHVCVD system and experimental procedure in the study are introduced. The, various analytic equipment to characterize the surface properties and crystal quality of Ge/InGaP/GaAs structures would be presented.
3.1 Chemical vapor deposition (CVD) system
A typical scheme of a CVD reactor is presented in Figure 3.1. A mixture of the precursor gases is diluted in a carrier gas (usually H2) and injected into a chamber, heated by a radio-frequency, infrared lamp or a resistance heater. The substrate is placed over a graphite
susceptor in the hot zone of the reactor. The precursor gases decompose after reaching a high temperature region and start to deposit on the substrate. The conditions of the flow dynamics, the chamber geometry, the precursor partial pressure and the operating pressure must be carefully chosen in order to promote an ordered deposition onto the substrate. Parasitic deposition on the reactor walls and heterogeneous reaction in the gas phase may hinder the crystal quality of the epilayer. Several commercial deposition systems are available on the market, but home-made reactors are also common in research institutes.
Today the most common technique to achieve Ge epitaxy is a CVD related process, with some variants such as metal-organic vacuum phase epitaxy (MOVPE), ultra-high vacuum CVD (UHV/CVD) or plasma enhanced CVD (PECVD). In conventional CVD, epitaxial growth is performed with partial pressures of water vapor and oxygen greater than 10-4 torr.
The majority of this water vapor and oxygen is due to outgassing from the walls of the chamber [11]. Contaminants such as oxygen and H2O lead to precipitates that can result in extended lattice defects such as stacking faults and microtwins. At growth temperatures below
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1000 °C there is high incorporation of these contaminants. These defects can cause
polycrystalline inclusions in the film or result in polycrystalline growth, leading to increased surface roughness [12]. UHV/CVD utilizes very low base pressures in the growth chamber to reduce the amount of H2O, O2, and other contaminants to which the wafer is exposed before and during the growth of epitaxial films.
3.1.1 Ultra high vacuum CVD (UHV/CVD) system
The machine used in this thesis is a multiple wafer UHVCVD reactor system. The growth system shown in Figure 3.2 consists of two chambers, a load lock chamber and a growth chamber where base pressure are under 10-7 torr and 10-9 torr respectively. The purpose of the load lock is to serve as an intermediary between the atmosphere and the deposition chamber, providing the isolation that ensures vacuum quality and integrity of the deposition chamber.
(a) Chamber system
The UHV/CVD growth chamber is made of quartz which is inserted into a furnace. One side of the growth chamber connects with two pumps which including a dry pump and a turbo pump. The other side of the quartz tube links load lock chamber which is constructed of stainless steel. The load lock chamber is also connected with the other turbo pump and a mechanical pump and has quick access door that enables loading the wafers. And the wafers can be put into the 4” quartz boat which is placed in the load lock. The growth chamber is resistance heated and is continuously pumped to keep UHV pressures all the time. The base pressure is as low as 10-9 torr, keeping the chamber practically free from contaminants.
(b) Transport system
The transport system has the transfer rod assembly consisting of a magnetically coupled and linear motion feedthrough. The linear motion feedthrough is used to locate the transfer rod precisely during the transferring and loading of the quartz boat.
18 (c) Gas system
Gases are introduced from the start of the chamber controlled by mass flow controllers (MFC) and vented at the end by the vacuum system consisted by mechanical and turbo pump.
The source gases used in UHV/CVD to grow Ge film is germane (GeH4).
3.1.2 Chemical reaction in UHV/CVD
In CVD growth three different regimes are recognized, that depend mainly on the growth temperature including thermodynamically limited growth, mass transport limited growth and surface kinetics limited growth. In the thermodynamically limited growth regime, which occurs at high temperatures above 800 °C, the deposition is mainly affected by the desorption of atoms from the growth surface. Mass transport limited growth is referred to as conventional growth regime and occurs roughly between 550-800 °C. In this regime, the growth rate and composition of the forming epitaxial layer is determined by the input partial pressure such as the flux of precursors. The growth temperatures in surface kinetics limited growth regime are lower than in the mass transport limited growth regime starting from about 600 °C and reaching as low as 400 °C. The surface adsorption rate of reaction source is lower than the diffusion rate of reactant source in the boundary layer. Therefore, in this region, the growth rate depends on the surface reaction rate and increases with increasing temperature. Ge layers are often grown at low growth temperature in UHV/CVD and that means the growth regime is the surface kinetics limited mode. Epitaxial growth will be discussed by the adsorption and decomposition of the hydride on the surface. The initial reaction for deposition of a species X is
XH₄ + 2 ∗→ HX₃ ∗ +H ∗ (3-1)
where XH4 is the hydride of the species X (Si or Ge), 2* represents two free surface sites and H* indicates a species bonded to a surface site. The following reaction is a series of reactions that further reduce the hydride. The final reaction is
XH ∗→ X ∗ +½H₂ (3-2)
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which results in the deposition of a film of species X on the surface. What is clear from these reactions is that the rate of deposition is limited by the amount of vacant surface sites. Most of these are being occupied by hydrogen from the initial reactions and thus it is the hydrogen desorption rate that ultimately decides the rate of film growth [13].
3.2 Ge film epitaxy in UHV/CVD
UHV/CVD operates at a very low pressure (base pressure of 10-9 torr) and a low temperature (~500°C). The effect of auto-doping, where dopants from the substrate diffuses into the epilayer and into the gas to be re-adsorbed later downstream, will be reduced. The system operated at low temperature also makes it possible to grow epitaxial films with very abrupt interfaces and doping profiles, which is useful for many device applications.
The low growth pressure means that growth rate is quite slow (~ 1-10 Å /min). But it also implies that the gas flow is molecular, with a molecule mean free path much longer than the reactor length. As mentioned above, turbulence effects are minimal and an even distribution of molecules over the samples can be assumed [14]. The throughput of UHV/CVD is high despite of the low growth rates, since simultaneous growth of multiple wafers is possible.
In many epitaxial systems, many problems would occur by impurity such as water vapor, oxygen and hydrocarbons. Due to the extremely low pressure in the UHV/CVD growth chamber and the nature of the turbo pumps the partial pressures of the impurities are
insignificant [15]. The only impurities which came from the source gases germane may not be completely pure. But due to the fact that the pressure during the growth is only about 10-3 torr, an impurity concentration of 1 ppm would only give a partial pressure on the order of 10-9 torr and should not be a big issue for the growth of epitaxial film. In this study, In0.5Ga0.5P/GaAs (100) wafer with 6°-offcut toward [110] wafers were used as substrates for Ge deposition.
Before Ge epitaxial deposition, the In0.5Ga0.5P/GaAs (100) wafer was cleaned by
NH4OH+H2O2+DI water (1:1:50) for 10 minutes, followed by HCl+H2O2+DI water (1:1:30) rinse for 1 minute [16], then loaded into the lock chamber. After the pressure of
load-20
lock reached 2×10-6 torr, the wafer was then transferred into the deposition chamber (main chamber) by the transfer rod. The wafer then went through a pre-bake step at 500°C for 5 minutes and the native oxide on the surface were removed in this step. During the Ge growth, the GeH4 flow rate was fixed at 10 sccm (in some cases 20 sccm), the pressure was controlled at 20 mTorr, and deposition time was varied to deposit undoped Ge on In0.5Ga0.5P/GaAs substrates.
3.3 Fundamental of characterization techniques
The epitaxial layers are analyzed by several parameters, including thickness, surface morphology, uniformity, dislocation density, and film quality. Each parameter can be characterized by one or more measurement techniques. Several common characterization techniques are used in this study, including scanning electron microscopy (SEM),
transmission electron microscopy (TEM), atomic force microscopy (AFM),
photoluminescence spectroscopy (PL), and X-ray electron spectroscopy (XPS). All the results will be briefly introduced and discussed.
3.3.1 Scanning electron microscopy (SEM)
Scanning electron microscopy (SEM) is similar to optical microscopy with exception that electrons are used instead of photons and the image is formed in a different manner, which will be described next. An SEM consists of an electron gun, a lens system, scanning coils, an electron collector, and cathode ray display tube (CRT). Electrons emitted from an electron gun pass through a series of lenses to be focused and scanned across the sample. The most common electron gun is a tungsten hairpin filament emitting electrons thermionically with an energy spread of around 2 eV. Tungsten sources have been largely replaced by lanthanum hexaboride (LaB 6) sources with higher brightness, lower energy spread (~ 1 eV) and longer life. Field emission guns are about 100× brighter than LaB 6 sources and 1000×
brighter than tungsten sources, respectively and energy spread of about 0.2 to 0.3 eV can be achieved with even longer lifetime than the other sources. The emitted electrons are
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accelerated through a voltage up to ~30 kV, and the resulting beam is finely focused by a series of magnetic coils to form a spot on the specimen. A scan generator moves this spot across the specimen via two sets of scan coils. The electrons that escape from the sample comprise the signal and can be collected by various electron detectors depending on the applications to monitor some emission (or property of) the specimen. The resultant signal is amplified and transferred to the display device.
accelerated through a voltage up to ~30 kV, and the resulting beam is finely focused by a series of magnetic coils to form a spot on the specimen. A scan generator moves this spot across the specimen via two sets of scan coils. The electrons that escape from the sample comprise the signal and can be collected by various electron detectors depending on the applications to monitor some emission (or property of) the specimen. The resultant signal is amplified and transferred to the display device.