Photoreflectance Signal Research Articles

Optical study of AlGaN/GaN based HEMT structures

A detailed photoluminescence (PL), time-resolved photoluminescence (TRPL), and photoreflectance (PR) analysis of AlGaN/GaN heterostructures grown on sapphire is presented in this paper. We have examined a number of samples in which the thicknesses of the Al0.3Ga0.7N layers vary from 18 to 75 nm. Room temperature PL under pulsed 266 nm excitation allowed for determination of the Al content in the examined thin AlGaN layers. Time-resolved PL measured at 1.6K exposed huge difference in the emission dynamics for different Al0.3Ga0.7N layer thickness. We observed an enormous increase of the emission decay above the critical thickness of the AlGaN layer. PR spectra (associated with the GaN main layer) measured on AlGaN/GaN systems are discussed in terms of the thickness of the capping layer. Photoreflectance analysis of the AlGaN/GaN interface showed that the phase of the signal shifts as the Al0.3Ga0.7N layer increases its thickness. These phase shifts of the PR signals related to GaN are interpreted in terms of optical interference effect in the AlGaN capping layer. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Investigations of AlGaN/GaN field-effect transistor structures by photoreflectance spectroscopy

Undoped AlGaN/GaN heterostructures with different content of AlGaN layer have been investigated by photoreflectance (PR) spectroscopy. PR resonances associated with the absorption in both GaN and AlGaN layers have been observed. The observed AlGaN-related transition has been identified as a band-to-band absorption in the layer being in under a strong built-in electric field. The electric field has been determined from the period of Franz-Keldysh Oscillations (FKOs) typical of layers being in a built-in electric field. It has been shown that the internal electric field in AlGaN layer increases by 21% (from ∼240 to 290kV/cm) with the rise of Al mole fraction from 20 to 27%. This phenomenon has been attributed to an increase in the surface potential for AlGaN layer due to the rise of Al mole fraction. In addition, PR signal related to GaN has been analyzed. It has been shown that the GaN-related part of PR spectrum exhibits FKOs, besides, sharp PR features which could be associated with an excitonic absorption far to the AlGaN/GaN, i.e. in the part of GaN layer being in weak internal electric field. The GaN-related FKOs have been attributed the band-to-band absorption in the GaN layer being in strong internal electric field, i.e. in the part of GaN layer close to the AlGaN/GaN interface. It has been determined that this field is in the range of 220–230kV/cm.

Photoreflectance investigations of HEMT structures grown by MBE

High electron mobility transistor (HEMT) structures consisting of: GaAs (buffer layer)/i-AlGaAs(spacer layer)/n-AlGaAs(barrier layer)/n-GaAs(cap layer) were grown by MBE on GaAs substrates. The optical properties of the HEMT structures were studied by photoreflectance (PR) spectroscopy at different temperatures. We observed in the PR spectra transitions located approximately at 1.41 and 1.85 eV, associated with the energy band edges of GaAs and AlGaAs, respectively. Oscillatory signals slightly above these energies were recognized as Franz–Keldysh oscillations (FKO). By employing lasers with different wavelengths as modulation sources, and by varying the thickness of the layers in the heterostructures we were able to identify the origin of the PR signals in the spectra. The FKO close to the GaAs band gap originate at the GaAs buffer/i-AlGaAs heterojunction where the two-dimensional electron gas is localized. Finally, low-temperature PR experiments indicate that the oscillatory signal observed above the AlGaAs band edge is formed by two overlapped signals, one of which originates in the GaAs(cap)/n-AlGaAs interface.

Temperature dependent photoreflectance and photoluminescence characterization of GaInNAs∕GaAs single quantum well structures

Ga 0.69 In 0.31 N x As 1 − x ∕ GaAs single quantum well (SQW) structures with three different nitrogen compositions ( x=0%, 0.6%, and 0.9%) have been characterized, as functions of temperature in the range 10–300K, by the techniques of photoreflectance (PR) and photoluminescence (PL). In PR spectra, clear Franz-Keldysh oscillations (FKOs) above the GaAs band edge and the various excitonic transitions originating from the QW region have been observed. The built-in electric field in the SQW has been determined from FKOs and found to increase with N concentration. The PR signal has been found to decrease for nitrogen incorporated samples when the temperature was lowered due to a weakening of the modulation efficiency induced by carrier localization. A careful analysis of PR and PL spectra has led to the identification of various excitonic transitions, mnH(L), between the mth conduction band state and the nth heavy (light)-hole band state. The anomalous temperature dependent 11H transition energy and linewidth observed in the PL spectra have been explained as originating from the localized states as a result of nitrogen incorporation. The temperature dependence analysis yields information on the parameters that describe the temperature variations of the interband transitions.

Photoreflectance study of p-type GaN layers

Layers of GaN:Mg ∼1.5 μm thick with different concentrations of Mg dopant have been investigated by room temperature photoreflectance (PR) spectroscopy. We have compared the results obtained by the PR technique with the results obtained by standard photoluminescence (PL) and reflectance (R) techniques. We have demonstrated that PR spectroscopy is an experimental technique which is sensitive to both optical and electrical properties of investigated samples, in contrast to PL and R spectroscopies, which are only sensitive to optical properties. This feature of PR spectroscopy is very useful for investigations of GaN:Mg layers, where we have to control both optical and electrical properties. In this paper we have shown that with the increase in the number of defect states in GaN:Mg layers, associated with the increase in Mg concentration, the PR signal decreases due to the weakening of the band bending modulation. This indicates that the electrical quality of the GaN:Mg layer deteriorates with the increase in the concentration of Mg atoms.

Photoreflectance characterization of GaAs/AlAs quantum wells with [formula omitted] super-flat interfaces grown by molecular beam epitaxy

GaAs/AlAs quantum wells (QWs) with effectively atomically flat heterointerfaces over a large area [“ (4 1 1) A super-flat interfaces”] grown on (4 1 1) A GaAs substrates by molecular beam epitaxy were characterized by room-temperature photoreflectance (PR) measurements. A larger PR signal corresponding to an optical transition between the 1st electron and the first heavy-hole levels (e1–hh1) was observed for the (4 1 1) A QW with a well width ( L w) of 3.2 nm compared to the QW simultaneously grown on a (1 0 0) GaAs substrate. However, spectral line width of the PR signal from the (4 1 1) A QW was comparable to that of the (1 0 0) QW. In contrast with this, larger and sharper PR signals of e2–hh2 and e3–hh3 transitions were observed for the (4 1 1) A QWs with L w =12.9 nm . The spectral line width of the room-temperature PR signal of the e n–hh n transition with higher quantum index ( n) is more sensitive to the interface flatness comparing to the low-temperature photoluminescence line width of the GaAs/AlAs QW.

Optical properties of GaInNAs/GaAs quantum wells: character of optical transitions and carrier localisation effect

The set of Ga 0.59 In 0.41 N x As 1-x GaAs single quantum wells (SQWs) with high nitrogen content (up to 5.2%) has been investigated in photoreflectance (PR) and photoluminescence (PL) spectroscopies at low and room temperatures. Strong manifestation of the carrier localisation effect due to the increase in nitrogen content has been observed. The carrier localization at low temperatures leads to a large Stokes shift and a decrease of the efficiency of band bending modulation. The second phenomena causes a significant decrease of PR signal at low temperatures. In addition, the temperature induced shift of the energy band gap significantly depends on nitrogen content. We have observed that the shift between 10 and 300 K is 82, 73, 67, and 62 meV for the SQW with the nitrogen content of 0, 2.0, 3.2, and 5.2%, respectively.

Influence of carrier localization on modulation mechanism in photoreflectance of GaAsN and GaInAsN

GaAs 0.98 N 0.02 and Ga0.95In0.05As0.98N0.02 layers have been investigated by photoreflectance (PR) and photoluminescence in 10–300 K temperature range. A decrease in PR signal has been found when the temperature was lowered. This effect is attributed to a weakening of modulation efficiency, which is induced by carrier localization that has been evidenced in low temperature photoluminescence. The Kramers–Kronig analysis is proposed as a simple method to determine the evolution of transition intensity with temperature when the change in the PR line shape can take place.

Photoreflectance and surface photovoltage spectroscopy characterisation of an InGaP∕InGaAsN∕GaAs NpN DHBT structure

An InGaP/InGaAsN/GaAs NpN double-heterojunction bipolar transistor (DHBT) structure has been characterised using the techniques of photoreflectance (PR), including the dependence of the signals on the polarisation {[1 1 0] and [1 1 0]} of the incident radiation, and surface photovoltage spectroscopy (SPS). The ordering parameter of the InGaP is deduced from the polarisation dependence of the PR signals from the emitter region. The observed Franz–Keldysh oscillations have been used to evaluate the electric fields in the collector and emitter regions. The field in the collector region agrees well with the theoretical value, while the field in the emitter region is found to be about 25 kV/cm smaller than the theoretical value not taking into account the possible ordering-induced screening effect. The difference is ascribed to the influence of the piezoelectric field related to ordering. In addition, the InGaAsN band gap is determined to be 1.196 eV by analysing the PR and SPS spectra in the base region. The narrower band gap of InGaAsN has led to a lower turn-on voltage, which shows great potential for the application of InGaAsN in low-power electronics.

Photoreflectance study in the E and E+Δ transition regions of GaP

Photoreflectance (PR) measurements have been carried out to determine the E0 and E0+Δ0 critical-point (CP) parameters in GaP at temperatures between 12 and 300 K using a He–Cd laser as modulation light source. The measured PR spectra provide distinct structures at ∼2.75–2.85 eV (E0) and ∼2.85–2.95 eV (E0+Δ0). These structures are successfully explained by a three-dimensional (3D) one-electron line shape plus an excitonic shape. The temperature dependence of the 3D CP and excitonic parameters (energy, amplitude, and broadening parameter) have been determined and analyzed using the Varshni equation and an empirical expression of Bose–Einstein type. The rapid decrease in the PR signal amplitudes is observed at T<100 K and can be explained by the weakened surface electric fields due to carrier freezing at such low temperatures. The 3D exciton binding energy at the E0/(E0+Δ0) edges of GaP has also been determined to be 13 meV.

Influence of laser pump density on the characteristic time constant and the intermediate-field electromodulation E 0 component of the photoreflectance signal

The influence of the laser pump density L on the intensity and the characteristic time constant of the intermediate-field electromodulation E 0 component of photoreflectance spectra in a direct-gap semiconductor was studied. The experiments were carried out using GaAs samples with carrier concentration n≈1016 cm−3 and laser pump densities in the range L=100 µW/cm2–1 W/cm2. For all of the samples under study, the logarithmic dependence of the intensity of the electromodulation signal on the laser radiation density was ascertained. No effect of the attendant variations of the characteristic time constant on the measured signal was observed.

PHOTOREFLECTANCE SPECTROSCOPY OF EXCITONS IN β-ZnP 2

Photoreflectance (PR) spectroscopy has been performed for the singlet exciton system of g -ZnP 2 in order to investigate fine structure of the higher exciton states. The PR spectrum shows clear structure at each exciton level with large modulation amplitude. It is shown that the mechanism by which the PR signal is produced is electroreflectance due to the periodic cancellation of the built-in surface electric field by photoexcited carriers. A high-resolution PR spectrum obtained by using a Ti:sapphire laser as a probe light reveals the higher members of singlet excitons up to n =6.

Photoreflectance Probing of Below Gap States in Gan/Algan High Electron Mobility Transitor Structures

ABSTRACTOptical Impedence Spectroscopy of GaN/AlGaN high electron mobility transistor structures (HEMTs) using photoreflectance exhibit a photoreflectance lags – the component of the modulated reflectance out of phase with the chopper – ranging from 0.1 to 0.5. Photoreflectance was performed using below gap pumping on various samples. Samples that do exhibit appreciable photoreflectance lag for above gap pumping show significantly enhanced photoreflectance signals for below gap pumping. Yet, samples that do not exhibit appreciable photoreflectance lag for above gap pumping do not exhibit a signal for below gap pumping. This implies that the photoreflectance phase lag is due to mid-gap trap states. At least 2 types of traps are found, above and below about 2.5 eV. This result means that that photoreflectance can be used as a probe of HEMT device quality.

Photoreflectance study of thermal degradation of n-InP/p+-InGaAs heterojunctions

The thermal stability of n-InP/p+-InGaAs heterojunctions was studied using photoreflectance (PR) spectroscopy. The PR signal amplitude from the n-InP depletion region decreased after postgrowth annealing. The reduction of the PR signal amplitude reflected an increase of recombination centers in the n-InP depletion region, which was attributed to the degradation of the adjacent carbon-doped p+-InGaAs layer. The reduction of the PR signal amplitude was more significant in samples with higher carbon doping concentration. We propose a model of the increase of the recombination centers that is consistent with our systematic experimental results, where the thermal degradation of the heavily carbon doped p+-InGaAs is taken into account. The present study shows that the PR method is quite helpful in the nondestructive diagnosis of the crystal quality of epitaxial wafers after thermal processing in heterojunction bipolar transistor device fabrication.

Photoreflectance study of δ-doped semiconductor layers by a fast Fourier transformation

Photoreflectance (PR) spectroscopy has been applied to the investigation of Si δ-doped GaAs, Al 0.35Ga 0.65As and AlAs layers grown by metal–organic vapor phase epitaxy (MOVPE) on GaAs substrates. The observation of Franz–Keldysh oscillations (FKO) and the application of fast Fourier transform (FFT) has allowed us to determine the internal electric field and, hence, the potential barrier between surface and δ-doped region of the layer. The FFT of the photoreflectance spectra has exhibited two separate heavy and light hole frequencies showing that the FKO in the PR signal are always the superposition of these two components.

Photoreflectance characterization of the plasma-induced damage in Si substrate

Si surface damage induced during Ar and CHF3/CF4/Ar plasma processing has been characterized by photoreflectance spectroscopy (PRS). The photoreflectance (PR) signal intensity decreases drastically as the rf input power increases in the Si substrate exposed to the plasma. The recovery of the plasma-induced damage is confirmed by PRS after subsequent annealing over 500 °C. We found, from the shift of the PR spectra, the strain at the Si surface induced by the plasma treatment might be due to the introduction of the ions, C, F, or H, not to those of Ar. The depth profile of the defect density in the Si substrate is estimated quantitatively by analyzing the depth profile of the PR signal intensities.

Correlation between photoreflectance signal intensity and current gain of InP/InGaAs heterojunction bipolar transistor structures

InP/InGaAs heterojunction bipolar transistor (HBT) wafers grown by metal–organic vapor-phase epitaxy were characterized by photoreflectance (PR) spectroscopy. We found that the intensity of PR signals from the InP emitter and InGaAs collector layers of the HBT wafer decreases with increasing emitter growth temperature and shows a linear positive correlation with the HBT current gain. On the other hand, the intensity of PR signals from the n-InP single layers scarcely changes with increasing InP growth temperature. Similar tendencies in the PR intensity were also observed with changes in postgrowth annealing temperature. The change in the PR intensity of the emitter and collector layers is expected to reflect the crystal quality of the adjacent InGaAs:C base layer, which determines the HBT current gain. The present PR method is eminently suitable for the nondestructive diagnostics of the crystal quality of InP/InGaAs HBT wafers.

Room temperature polarized photoreflectance characterization of GaAlAs/InGaAs/GaAs high electron mobility transistor structures including the influence of strain relaxation

Using room temperature photoreflectance (PR) we have characterized the properties of three GaAlAs/InGaAs/GaAs high electron mobility transistor structures with two different well widths fabricated by molecular beam epitaxy on (001) GaAs substrates. The samples were denoted as #1, #2, and #3 with well widths of 140, 160, and 160 Å, respectively. Samples #2 and #3 were grown on substrates with different threading dislocation densities. For the latter two samples the well width exceeds the pseudomorphic limit so that there are some strain relaxation and related misfit dislocations as determined from the x-ray measurements. In order to detect the anisotropic strain of the misfit dislocations related to strain relaxation, the PR measurements were performed for incident light polarized along [110] and [11̄0] directions. Evidence for the influence of the strain relaxation upon the relaxed channel was provided by the observed anisotropy of the polarized PR signal in the InGaAs channel layer. Signals have been observed from every region of the sample, making it possible to evaluate the In and Al compositions, channel width and two-dimensional electron gas density, as well as the properties of the GaAs/GaAlAs multiple quantum well buffer layer.

MBE growth and RHEED characterization of MnSe/ZnSe superlattices on GaAs (1 0 0) substrates

[(MnSe) x /(ZnSe) y ] 60 superlattices ( x=4–16 Å, y∼12 Å) are grown on GaAs(1 0 0) substrates by molecular beam epitaxy (MBE) and are characterized by in situ reflection high-energy electron diffraction (RHEED), X-ray diffraction (XRD), photoluminescence (PL) and photoreflectance (PR). MnSe in the superlattice structure is confirmed to be zincblende (ZB) by RHEED analysis. Lateral lattice spacings a 0 1 ̄ 1 of the individual MnSe and ZnSe layers in the superlattice are measured from the interval of the RHEED streaks. The lateral lattice spacing of the superlattice with MnSe layers thinner than 4 Å is fixed to that of the ZnSe buffer layer throughout the 60-period superlattice growth, that is, the superlattices grow coherently to the ZnSe buffer layer. Superlattices with thicker MnSe layers show increases in the lateral lattice spacing as the growth proceeds, and the RHEED pattern becomes spotty. Coherently grown samples show PL and PR signals due to excitonic transitions in the MnSe/ZnSe type-I superlattice. A small Stokes shift of 2 meV indicates the high quality of the superlattices. The energy gap of the ZB MnSe is estimated to be 3.4 eV at 13.5 K. Mn-related luminescence bands are observed at 2.1 and 1.7 eV, and the intensity of the 1.7 eV band is very small in coherently grown samples.

Unpinned behavior of the surface Fermi level of GaN detected by photoreflectance spectroscopy

The photoreflectance signal from GaN films is sensitive to the ambient medium. A large decrease in the photoreflectance amplitude is observed, when the ambient medium is changed from air to vacuum. This effect is attributed to ultraviolet-light-induced desorption of oxygen from the sample surface leading to a reduction of the surface barrier height. The effect is absent, when a thin Ti layer is deposited on top of the GaN film. A simple model is used to demonstrate that the surface photovoltage can be strongly reduced with a decrease of the surface barrier height.