Ga Desorption Rate Research Articles

Effects of GaN template annealing on the optical and morphological quality of the homoepitaxially overgrown GaN layer

Growth of GaN based devices such as light emitting diodes and laser diodes often occurs on already prepared GaN templates grown on native (e.g. bulk GaN) and non-native (e.g. sapphire) substrates. High temperature annealing methods that are generally used for growth on other substrates cannot be completed on GaN templates due to the decomposition at temperatures above 800 °C in hydrogen rich ambient. The decomposition rate is higher than the Ga desorption rate from the surface under most conditions and causes Ga droplets to form on the surface which can hinder further growth. The effect of the annealing conditions on the GaN homoepitaxy on GaN templates is presented. In-situ annealing conditions were developed for annealing the GaN templates to remove any contamination while preventing significant Ga droplets from accumulating on the surface in both H2 and N2 environments to obtain optimized annealing conditions. As measured by AFM the RMS roughness of the GaN surfaces increases upon annealing, but becomes similar to the starting template after 1 μm, indicating complete recovery. The optical quality of the homoepitaxially overgrown layers as measured by PL shows improved band edge luminescence and band edge to yellow band ratio for annealing in H2 while it degrades for annealing in N2 only. Hall measurements show superior characteristics for annealing in H2 as well. These results will be especially important because bulk substrates are subjected to polishing damage that is known to negatively affect the surface condition and initial growth nucleation.

GaN decomposition in H 2 and N 2 at MOVPE temperatures and pressures

GaN decomposition rates were measured in H 2, N 2, and mixed H 2 and N 2 flows for pressures and temperatures typically encountered in metalorganic vapor phase epitaxy. The rates for GaN decomposition, Ga desorption, and Ga droplet accumulation, were obtained from weight measurements before and after annealing the GaN films in a close-spaced showerhead reactor. In H 2 at constant temperature, the GaN decomposition rate is enhanced when the reactor pressure is greater than 100 Torr. Unlike H 2, the decomposition rate in N 2 did not change as a function of pressure. The enhanced GaN decomposition rate in H 2 is not due to an increase in the Ga desorption rate, which is constant vs. pressure, but instead is due to H 2 dissociation on the surface followed by NH 3 formation and desorption. NH 3 formation is suggested by the cubic decrease in the GaN decomposition rate as N 2 is substituted for H 2. The measured activation energies, E A, for the GaN decomposition range from 0.34 to 3.62 eV and depend strongly on the annealing conditions. By comparing measured and literature values of the E A, four distinct groupings of the E A are observed. The four distinct groupings of the E A imply that there are possibly four different reactions which limit the GaN decomposition rate. Connections between the GaN decomposition and improved GaN growth are discussed. This includes a discussion of changes that occur in the nucleation layer during the ramp from low to high temperature, as well as increases in GaN grain size as the growth pressure is increased.

Possibility of a quasi-liquid layer of As on GaAs substrate grown by MBE as observed by enhancement of Ga desorption at high As pressure

The As vapor pressure dependence of the Ga desorption rate during molecular beam epitaxy (MBE) growth on GaAs( n11)A ( n=1–4 hereafter) substrates was studied by photoluminescence (PL) measurements at 12 K for undoped AlGaAs/GaAs asymmetric double quantum wells (ADQWs). Reflection high energy electron diffraction (RHEED) oscillation measurements on a GaAs(100) surface were also used. Two K-cells of As solid sources (corresponding to beam equivalent pressures (BEPs) of 9.0×10 −6 and 4.5×10 −5 Torr) were used to change the As pressure rapidly. The Ga flux and substrate temperature were kept constant at 0.76 ML/s and 12 K, respectively, while the As flux changed from 7.6 (BEP 9.0×10 −6 Torr) to 32 ML/s (4.5×10 −5 Torr). With increasing As pressure, two separated PL peaks for the wide well (WW) of high index substrates were observed. This peak separation is attributed to a reduced well depth from an increasing Ga desorption rate. The energy differences of the PL peak depending on the off-angle from (111)A to (100) plane indicates an orientation-dependent Ga desorption rate. Moreover, amongst all ( n11)A and (100) planes, the Ga desorption rate was smallest from the (111)A surface. The increase of Ga desorption from the surface at high As pressures probably arose from an increasing coverage with a quasi-liquid layer (QLL).

Enhanced GaN Decomposition at MOVPE Pressures

GaN decomposition was studied above 800 °C in flowing H2 and N2 for pressures ranging from 10 to 700 torr. From careful weighings of the GaN film on sapphire before and after annealing, the rates for GaN decomposition, Ga surface accumulation, and Ga desorption were obtained. An enhancement in the GaN decomposition rate was observed in H2 pressures greater than 100 torr. Even with this enhanced GaN decomposition, the Ga desorption rate is nearly constant at higher pressures. As a result, Ga droplets accumulate on the surface. For N2 pressures ranging from 76 to 400 torr no net enhancement in the GaN decomposition rate is observed and the GaN decomposition rate is reduced compared to identical annealing conditions in H2. This suggests that H2 is acting chemically to reduce the barrier for GaN decomposition. This may occur through a surface mediated dissociation of H2 followed by the formation of more mobile and volatile hydrogenated N and Ga species. The significance of this study for GaN growth is that by increasing the GaN decomposition, the Ga atoms diffuse farther and subsequently re-incorporate into the growing lattice, increasing the GaN crystal quality. Connections between the enhanced GaN decomposition rate and the coalescing of nucleation layer during the ramp to high temperature and the consequences for the high temperature growth are discussed.

Enhanced GaN decomposition in H2 near atmospheric pressures

GaN decomposition is studied at metallorganic vapor phase epitaxy pressures (i.e., 10–700 Torr) in flowing H2. For temperatures ranging from 850 to 1050 °C, the GaN decomposition rate is accelerated when the H2 pressure is increased above 100 Torr. The Ga desorption rate is found to be independent of pressure, and therefore, does not account for the enhanced GaN decomposition rate. Instead, the excess Ga from the decomposed GaN forms droplets on the surface which, for identical annealing conditions, increase in size as the pressure is increased. Possible connections between the enhanced GaN decomposition rate, the coarsening of the nucleation layer during the ramp to high temperature, and increased GaN grain size at high temperature are discussed.

Enhanced GaN Decomposition at Movpe Pressures

AbstractGaN decomposition was studied above 800°C in flowing H2 and N2 for pressures ranging from 10 to 700 torr. From careful weighings of the GaN film on sapphire before and after annealing, the rates for GaN decomposition, Ga surface accumulation, and Ga desorption were obtained. An enhancement in the GaN decomposition rate was observed in H2 pressures greater than 100 torr. Even with this enhanced GaN decomposition, the Ga desorption rate is nearly constant at higher pressures. As a result, Ga droplets accumulate on the surface. For N2 pressures ranging from 76 to 400 torr no net enhancement in the GaN decomposition rate is observed and the GaN decomposition rate is reduced compared to identical annealing conditions in H2. This suggests that HI- is acting chemically to reduce the barrier for GaN decomposition. This may occur through a surface mediated dissociation of H2 followed by the formation of more mobile and volatile hydrogenated N and Ga species. The significance of this study for GaN growth is that by increasing the GaN decomposition, the Ga atoms diffuse farther and subsequently re-incorporate into the growing lattice, increasing the GaN crystal quality. Connections between the enhanced GaN decomposition rate and the coalescing of nucleation layer during the ramp to high temperature and the consequences for the high temperature growth are discussed.

Gallium desorption behavior at AlGaAs/GaAs heterointerfaces during high-temperature molecular beam epitaxy

A Monte Carlo simulation study is performed to investigate the Ga desorption behavior during AlGaAs-on-GaAs heterointerface formation by molecular beam epitaxy. The transients in the Ga desorption rate upon opening the Al shutter are shown to be associated with the concurrent reduction in the V/III flux ratio. Monte Carlo simulations employing a constant V/III flux ratio yield a “steplike” variation in the Ga desorption rate with the resulting interfaces closer in abruptness to the ideal AlGaAs-on-GaAs interface. Further details on the stoichiometry of the interface and its relationship with predicted Ga desorption profiles is presented.

The origin of gallium desorption transients during AlGaAs/GaAs heterointerface formation by molecular beam epitaxy

A Monte Carlo simulation study is performed to investigate the Ga desorption behavior at AlGaAs/GaAs heterointerfaces during growth by molecular beam epitaxy. The transients in Ga desorption rate upon opening/closing the Al shutter arise due to both the change in V/III flux ratio and the Al–Ga surface exchange mechanism. It is shown that the desired “steplike” variation in Ga desorption rate at each interface can be attained by a growth procedure employing a constant V/III flux ratio.

A Monte Carlo study of gallium desorption kinetics during MBE of heterostructures

Abstract In situ desorption mass spectrometry studies (DMS) of AlGaAs GaAs molecular beam epitaxy (MBE) at high substrate temperatures reveals a rich transient behavior in the Ga desorption signal during the formation of the AlGaAs-on-GaAs interface. In this study we develop Monte Carlo (MC) models for Ga desorption, in which the effects of the AlGa interaction strength and the inclusion of an AlGa surface exchange mechanism are investigated. The models which best describe the experimental observations are identified. The transients in Ga desorption rate at the AlGaAs-on-GaAs interface and the experimentally observed reduction in Ga desorption energy during growth of AlGaAs are explained in terms of the reduction in V III flux ratio accompanying the opening of the Al shutter. The proposed effects of this reduction in V III flux ratio are consistent with the results predicted by the different models.

Substrate temperature change in III–V molecular beam epitaxy

The surface temperature (T s ), as inferred by desorption mass spectrometric determination of the Ga desorption rate, was followed in time after instantaneous changes were made in the setpoint temperature (T stpt ) during molecular beam epitaxy (MBE) growth of GaAs. Comparison between the time-evolution of T s and that of the non-contact thermocouple-indicated temperature (T TC ) shows that large T s overshoot is likely to occur under typical MBE growth conditions. Such T s overshoot can lead to significant compositional grading at nominally abrupt heterojunctions. Details of how T s evolves in time are found to depend on substrate conductivity, temperature controller settings (proportional band, integral, derivative), and maximum heater current setpoint.

Reliable substrate temperature measurements for high temperature AlGaAs molecular-beam epitaxy growth

We describe a reproducible and precise temperature control system for the molecular beam epitaxial growth of AlGaAs at substrate temperatures in the 700 °C range. Coating of the optical pyrometer window was observed to dramatically affect the pryometer readings making it an unreliable temperature measurement technique. By observing pyrometer interference oscillations during both GaAs and AlGaAs epitaxy, the Ga desorption rate during AlGaAs growth was estimated. The known temperature dependence of the Ga desorption rate allowed the substrate temperature to be calculated, and corrected, when necessary, during the growth. This method allowed the absolute AlGaAs substrate temperature to be controlled within ±3 °C over a six month production cycle.

Desorption mass spectrometric control of composition during MBE growth of AlGaAs

We report on the implementation of technique of desorption mass spectrometry (DMS) in a real-time feedback loop for continuous monitoring and control of layer composition during molecular beam epitaxy growth of AlGaAs on (001) GaAs at high substrate temperatures. With this technique, the Ga desorption rate F d(Ga) and thus the composition is monitored continuously and kept at the desired value by adjusting either the substrate heater power or the incident arsenic flux. Composition control by DMS during growth of nominally constant composition layers is found to be superior to that obtained with conventional MBE growth, as indicated by F d(Ga) versus time profiles and low temperature photoluminescence (PL) results. The ability to produce complex graded structures by DMS control is demonstrated by the growth of nominally parabolic AlGaAs quantum wells, for which photoreflectance and PL results are consistent with the desired structure. The technique is quite general and applicable to many different compounds.

RHEED studies of Ga desorption from GaAs and of As desorption from Si-doped GaAs during growth interruption

We obtain accurate measurements of Ga desorption rate under conditions of Knudsen evaporation between 600 and 630°C from (001) GaAs or A1As using only RHEED. Within two monolayers from an A1As interface the Ga desorption is reduced by a factor of 2.5−3. In another experiment we observe that a partial surface coverage of a GaAs surface by Si has no effect on Ga desorption but increases strongly the As desorption, leading accordingly to a lowering of the transition temperature to a Ga-rich surface.

Arbitrary Composition Profiles by MBE Using Desorption Mass Spectrometry

ABSTRACTThe ability to produce semiconductor structures with arbitrary vertical composition profiles offers the device physicist new freedom in device structure design. We report on the incorporation of the in-situ sensor technique of desorption mass spectrometry (DMS) in a realtime feedback loop for the continuous control of desorption rates. Assuming previous knowledge of relevant incident fluxes, this technique provides for continuous determination of the composition of the outermost layer of the growing crystal. Using AlGaAs growth as an example, the Ga desorption rate Fd(Ga) is monitored continuously during growth, providing an accurate measure of Al-fraction in the topmost layer of the growing film. By comparing the measured and desired Fd(Ga), an error signal is generated and minimized by varying an appropriate growth parameter. Structures which are vertically graded in composition are produced by programming a varying Fd(Ga) versus time profile. Both substrate temperature Ts and incident arsenic flux Fi(As2) affect Fd(Ga) in a controllable way. Thus, two distinct control loops are constructed based on variable Ts and Fi(As2), respectively. As demonstration vehicles, graded AlGaAs structures with linear, parabolic, sawtooth, and sinusoidal compositional profiles have been produced and characterized. Additionally, we discuss some of the limitations of this technique and its applicability to other materials.

Gallium desorption from (Al,Ga)As grown by molecular beam epitaxy at high temperatures

We report on a detailed study by cross-sectional transmission electron microscopy (XTEM) of gallium desorption from (Al,Ga)As structures grown by molecular beam epitaxy (MBE) at substrate temperatures in the range 680–730°C. The Ga desorption rate ( D r) depends only on substrate temperature, with an activation energy, E a for re-evaporation of 2.56 eV, comparable to E a for Ga evaporation from liquid gallium. The presence of aluminum has no measurable influence on D r except where the desorbing gallium flux exceeds the incident flux ( D rå G r), when a few monolayers of residual GaAs can be detected on an AlAs surface. No As 4 overpressure dependence has been observed. In practice, therefore, multilayer structures of (Al,Ga)As with controlled thicknesses and compositions can be grown with As 4 in the temperature regime investigated by making a constant allowance for D r, irrespective of the compositional fraction of the (Al,Ga)As.

Impact of MBE-growth rate on optical properties of AlGaAs/GaAs quantum well structures

The results of the first systematic study of the correlation of the light emission from GaAs QW's, 2 nm, 5 nm and 8 nm wide, on the growth rate r in the range from 0.1 μm/h to 1 μm/h at a growth temperature of 620 °C are reported here. Emission energy positions, lineshapes and intensities are found to strongly vary with the growth rate. In particular, we observe that nonstandard low growth rates lead to much less interface roughness, a reduction of impurity and trap concentrations and higher quantum efficiency. For low growth rates the Ga desorption rate depends on growth rate. For higher growth rates it saturates due to the build up of an equilibrium vapor pressure at the surface.