Band Edge Thermometry Research Articles

Growth-induced temperature changes during transition metal nitride epitaxy on transparent SiC substrates

Noncontact band edge thermometry based on diffuse reflectance was used to monitor and control the substrate temperature rise during the MBE growth of niobium nitride transition metal nitrides on transparent, wide-bandgap silicon carbide substrates. Temperature transients as large as 135 °C are induced by changing the main substrate shutter state. The growth of niobium nitride films as thin as ∼5 nm leads to temperature increases as large as 240 °C. In addition, a temperature decrease during the growth of ultrawide-bandgap AlN films on niobium nitride was observed and characterized. The causes of the observed temperature excursions are explained by considering the Stefan–Boltzmann law, and ways to better control the substrate temperature are discussed.

Band edge thermometry for the MBE growth of (Hg,Cd)Te-based materials

The material system (Hg,Cd)Te is a prototype system for topological insulators and Dirac semi-metals. Molecular beam epitaxy is the preferred method for the growth of (Hg,Cd)Te epilayer heterostructures and CdTe-HgTe core-shell nanowires. Unfortunately, the high crystalline quality desired for charge transport investigations is only achieved for a very narrow window of the growth temperature. Additionally, the low growth temperature requires a special thermometry. Here, we demonstrate the improvement of the growth of (Hg,Cd)Te epilayer heterostructures and CdTe nanowires by the use of band edge thermometry. We show that even for narrow-gap materials band edge thermometry provides a suitable method to control the temperature directly at the growth front with a high precision and reproducibility, in contrast to a conventionally used thermocouple.

In Situ Band-Edge Monitoring of Cd1−yZnyTe Substrates for Molecular Beam Epitaxy of HgCdTe

We demonstrate the benefits of in situ band-edge monitoring of CdZnTe substrates for molecular beam epitaxy (MBE). The production of large-area Cd1−yZnyTe(211)B substrates up to and exceeding 8 × 8 cm2 brings new challenges from the perspective of MBE growth of Hg1−xCdxTe. Localized variation of the Zn concentration, even less than y = 0.01, can potentially cause variation in lattice-misfit with the HgCdTe epi-layers potentially resulting in ∼ 10 × difference in etch-pit density. Variation in Zn concentration is expected to be greater over larger substrates, and thus local differences in dislocation density may also be expected. The substrate Zn concentration has been determined using in situ band-edge measurement prior to nucleation of HgCdTe. Ex situ infrared transmission spectra has been used to confirm the validity of in situ band-edge wavelength measurements. We also examine the usefulness of band-edge thermometry (BET), for monitoring substrate temperature during Te/oxide desorption and for nucleation of HgCdTe epi-layers. In principle, knowledge of the exact Zn content across the largest area of the substrate, allows for tuning growth parameters to maximize yield of perfectly lattice-matched material.

Development of a Long-Wave Infrared Band-Edge (LWIR BE) thermometry instrument.

Accurate measurement of substrate temperature is one of the most critical process control parameters for molecular beam epitaxy (MBE) growth. Band-edge thermometry instruments have proven to be a valuable tool for process control during MBE growth of semiconductor films, providing as high as ±1 °C temperature resolution. The increasing use of InAs, GaSb, and AlSb iii-v materials necessitates a method for accurately measuring the temperature of their closely lattice-matched GaSb substrates. Current-technology instruments typically rely on InGaAs detector materials which have a maximum wavelength λ detection of ∼1.7 μm, but GaSb substrates have a band gap energy corresponding to λ > 2 μm. A band-edge thermometry instrument capable of λ > 2 μm has been developed using an InAs/InGaSb strained-layer superlattice detector sensitive to 2-9.5 μm long-wave IR wavelengths.

Temperature monitoring of narrow bandgap semiconductors

An integrated spectral pyrometry (ISP) technique particularly well suited to monitor the temperature of small bandgap semiconductors during molecular beam epitaxial growth is proposed. The technique relies on integrating the thermal radiation power emitted by the wafer over a spectral range where it is fully opaque, so as to avoid contribution from radiation transmitted from the substrate heater. In the present work, a 900–1700 nm array InGaAs spectrometer normally employed for band-edge thermometry was used. The temperature dependence of the integrated signal can be expressed by an Arrhenius-type exponential relation. The calibration procedure and the method employed to compensate for the background radiation from other hot objects in the reactor is discussed. The ISP performance is then demonstrated by monitoring the temperature during the growths of a Si doped InAs layer on InAs substrate and of an InSb/AlInSb quantum well structure on GaAs substrate. The ISP technique allows reproducible wafer temperature monitoring down to about 200 °C.

In situ thickness and temperature measurements of CdTe grown by molecular beam epitaxy on GaAs substrate

An in situ technique based on diffuse reflectance spectroscopy to measure the thickness and temperature of CdTe films grown by molecular beam epitaxy on GaAs substrates is studied in this paper. A new model considering the interference of a thin film is set up to correct the previous errors observed in temperature measurements. A precise thickness parameter can be acquired by fitting the curves of the diffuse reflectance spectrum. This correction procedure is verified at the growth temperature of CdTe, which shows a nonoscillation result of the band edge thermometry. Compared with the thickness measured by an ex situ IR transmission spectrum, the thickness determined by using this model has an accuracy of less than 0.01 μm. This measurement can be used for precise in situ noncontact precise monitoring of growth temperature and thickness.

Short wavelength band edge thermometry during molecular beam epitaxial growth of GaN on SiC substrates and detected adatom self-heating effects

Band edge thermometry was used to measure the temperature of 4H SiC substrates, band gap of 3.25 eV, during heat-up followed by molecular beam epitaxial growth of GaN structures. The surface of the rotating wafer was illuminated with a white light source which provided sufficient scattered signal for SiC substrate temperature measurements of single side polished substrates and unmetallized double side polished substrates. Substrate temperatures above 400 °C could be measured with good signal-to-noise ratio. At 750 °C the precision was ±1 °C despite the small temperature dependence of the SiC band gap. A complete spectral interference oscillation was observed for a layer thickness of 800 Å. The scattered intensity was found to be dependent on whether gallium- or nitrogen-rich growth conditions were used. During conventional gallium-rich GaN growth on unmetallized, double sided polished substrates, the thermometer detected an increase in wafer temperature due to the presence of surface gallium. A similar adatom self-heating effect was observed with indium.

Combined molecular beam epitaxy low temperature scanning tunneling microscopy system: Enabling atomic scale characterization of semiconductor surfaces and interfaces

A molecular beam epitaxy and low temperature scanning tunneling microscopy chamber have been integrated to characterize both compound and elemental semiconductor surfaces and interfaces. The integration of these two commercially available systems has been achieved using a custom designed sample transfer mechanism. The MBE growth chamber is equipped with electron diffraction and provides substrate temperature measurements and control by means of band-edge thermometry accurate to within ±0.5°C. In addition, the microscope can operate at temperatures as low as 4K and perform ballistic electron emission microscopy measurements. The chamber that houses the microscope includes a preparation chamber with an evaporation source for metals. The entire STM chamber also rests on an active vibration isolation table, while still maintaining an all ultrahigh vacuum connection to the MBE system.

Growth related interference effects in band edge thermometry of semiconductors

During epitaxial growth on GaAs and InP substrates two effects were observed that can interfere with correct substrate temperature measurements when using band edge thermometry. The first effect was due to specular reflection from a hot effusion cell opposite the optical detector. The worst case observed was a false temperature drop of ∼30°C upon starting growth. This effect was eliminated by moving the detector to a port that doesn’t allow specular reflection from any cell. The second effect is due to the development of constructive and destructive interference fringes while growing heterostructures. False temperature oscillations as large as ±5°C were seen when growing a thick AlGaAs∕GaAs heterostructure on GaAs. The solution to reducing the false TS oscillations is the use of a third derivative of the spectra, making the location of the band edge much less susceptible to interference effects. This technique reduced the false oscillations to ±1.5°C. However, the second interference effect also opens up new possibilities for adapting band edge thermometry to extracting real time growth rate and composition information by monitoring oscillations at several fixed wavelengths both above and below the band edge.

In situ temperature control of molecular beam epitaxy growth using band-edge thermometry

Band-edge thermometry is becoming an established noncontact method for determining substrate temperature during molecular beam epitaxy. However, with this technique thin-film interference and/or absorption in the growing epilayer can cause shape distortions of the spectrum that may be interpreted erroneously as real temperature shifts of the substrate. An algorithm is presented that uses the width of the spectrum to correct for apparent temperature errors caused by interference and absorption in the epilayer. This correction procedure is tested on substrate temperature data taken during the growth of a λ=930 nm resonant cavity, where the apparent substrate temperature oscillates ±5 °C during the growth of the mirror stacks. These oscillations are reduced to ±3 °C using the correction algorithm. A recently developed model for the substrate temperature dynamics in molecular beam epitaxy shows that roughly ±1 °C of the remaining ±3 °C temperature oscillations are real. Band-edge thermometry is also used to control the substrate temperature to within ±2 °C during the growth of near-lattice-matched InGaAs on InP, whereas the same growth under constant thermocouple temperature would result in a 50 °C rise in the actual substrate temperature.