A comprehensive diagram to grow InAlN alloys by plasma-assisted molecular beam epitaxy

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InAlN alloys have a direct band gap tunable from 0.7 eV to 6.2 eV, and, for a 17% In there is lattice match (in-plane) to GaN.Heterostrutures and devices including InAlN layers, such as resonant cavities, multi-quantum wells for high-speed inter-subband devices, or high electron mobility transistors have been recently reported [1][2][3][4].
Growth diagrams, useful to identify optimum conditions, have been established for binaries (AlN, GaN, InN), grown by plasma-assisted molecular beam epitaxy (PA-MBE), from the surface morphology dependence on growth temperature and impinging fluxes [5][6][7][8].However, the growth of InAl(Ga)N alloys poses much more difficulties due to InN thermal decomposition [9][10][11][12] and strong differences between InN and AlN.This work reports on the growth and characterization of metal-face InAlN layers on GaN templates.Indium incorporation and surface morphology are analysed as a function of growth temperature and metal fluxes to build up a growth diagram.
Growth temperature was measured with an Ircon Modline3 optical pyrometer.Metal fluxes (ΦGa, ΦAl, ΦIn), measured as Beam Equivalent Pressure (Bayard Alpert), were calibrated in atoms/sec•cm 2 using cross-sectional scanning electron microscopy (SEM) data from N-rich GaN, AlN and InN thick layers grown at temperatures where thermal decomposition and adatoms desorption are negligible [5].Similarly, the active N flux, ΦN, was calibrated in atoms/sec•cm 2 using cross-sectional SEM data from Ga-rich GaN thick layers grown at low temperatures (680 ºC).Prior to the InAlN growth, a 100 nm thick GaN buffer layer was grown at 700 ºC under intermediate Ga-rich conditions [5] to obtain a smooth and flat surface.Alloy compositions were assessed by high resolution x-ray diffraction (HR-XRD) and surface morphologies were characterized by SEM (JEOL JSM-5800) and by atomic force microscopy (AFM, Digital Instruments MMAFM-2).
To analyse separately the effects of growth temperature and impinging In flux on In incorporation two sets of samples were grown.In a first set (Series-A) all impinging fluxes were kept constant, and the growth temperature varied between 450 and 610ºC, a range where Al sticking coefficient is 1, being preferentially incorporated due to a much higher energy of the Al-N bond than that of In-N [9,11].The III/V flux ratio was <1 (Nrich) with impinging fluxes of ΦAl = 2.7 ± 0.1 x 10 14 atoms/s•cm 2 , ΦIn = 1.2 ± 0.1 x 10 14 atoms/s•cm 2 and ΦN = 4.2 ± 0.1 x 10 14 atoms/s•cm 2 .A 0.31 ± 0.03 nominal InN mole fraction is derived from ΦIn/(ΦIn+ΦAl) [9].In Series-B the growth temperature was set at 535 ºC and ΦIn was varied while keeping the same ΦAl and ΦN of Series-A.In-face InN [7].
As in InGaN alloys [9], we may expect that, under steady state conditions, InN losses ( losses InN Φ ) by thermal decomposition are proportional to the Boltzmann factor with an activation energy, Ea, and to [InN] * with a constant factor C: The nominal In incorporation rate, inc  As temperature increases, more In excess is available, thus, less impinging ΦIn is required for stoichiometry conditions.Above this solid line, the In excess cannot be neither desorbed (below ~560 ºC) nor incorporated to the crystal because of N shortage, thus, it accumulates on the surface as droplets.The stoichiometry condition determined by this solid line is given by: For temperatures above 560 ºC, In desorption becomes non-negligible, opening a window in which growth can proceed under intermediate In-rich conditions [9,11,12].
As in the case of In(Al)GaN alloys the best quality is expected for those samples grown in this regime due to the presence of a surfactant In adlayer with coverages ranging from zero (lower N-rich boundary) to 2.5 monolayers (upper In-droplets boundary) [7,9,[11][12][13].Within this window (between the two solid lines) the required ΦIn increases with temperature to compensate In desorption.The lower boundary (towards N-rich) represents strict stoichiometry, whereas the upper one (towards In-droplets) is slightly metal rich.Both solid lines follow the expression: where ) (T des In

Φ
reaches its maximum value (for a given temperature) at the upper boundary.These maximum values follow an Arrhenius temperature dependence with an activation energy of 2.49 eV [7].The region where In incorporation is negligible, arises from InN-losses higher than the In incorporation rate limited by the impinging fluxes.
The diagram in figure 2 clearly shows that, for a given set of metals and N fluxes, the alloy composition changes significantly with the growth temperature.In addition, for a given temperature, the alloy composition is also expected to change with ΦIn outside the In-droplets regime.Figure 3

Figure 1 (
Figure 1(a) shows a continuous decrease of the actual InN mole fraction, [InN] * ,

InΦ
, within the 500-585 ºC temperature range (In droplets region) when decomposition and desorption are not considered (ideal case), equals (ΦN -ΦAl) [9].When decomposition and desorption are considered, an actual In incorporation rate can be defined, inc In Φ * , which relates to the [InN] * value.The losses InN Φ were estimated within this temperature range as the inc In Φinc In Φ * difference being the latter derived from HR-XRD data: [ ] InN] * values, that yield C and Ea values [best fit to eq.(1)] of 1.27 x 10 27 InN/s•cm 2 and 2.0 eV, respectively.This energy value is in good agreement with both the In-N bond energy, 1.93 eV[14], and the activation energy for thermal decomposition of In-face InN, 1.92 eV[7].

Figure 2
Figure 2 shows a growth diagram for metal-face InAlN determined from the shows the increase of [InN]* as a function of ΦIn for Series-B.When stoichiometry is reached at the growth front (solid line boundary in figure 2) [InN]* reaches its maximum value, beyond which, In droplets develop while [InN]* remains constant.

Figure 4
Figure4shows the characteristic surface morphologies for samples grown at

Figure 2
Figure 2The PA-MBE growth diagram for metal-face InAlN.Four different growth

Figure 4
Figure 4Representative surface morphologies of InAlN samples grown within the different growth regimes with ΦAl = 2.7 x 10 14 atoms/s•cm 2 and ΦN = 4.2 x