The low-temperature mobility of two-dimensional electron gas in AlGaN/GaN heterostructures

To reveal the internal physics of the low-temperature mobility of two-dimensional electron gas （2DEG） in Al- GaN/GaN heterostructures, we present a theoretical study of the strong dependence of 2DEG mobility on Al content and thickness of AlGaN barrier layer. The theoretical results are compared with one of the highest measured of 2DEG mobility reported for AlGaN/GaN heterostructures. The 2DEG mobility is modelled as a combined effect of the scat- tering mechanisms including acoustic deformation-potential, piezoelectric, ionized background donor, surface donor, dislocation, alloy disorder and interface roughness scattering. The analyses of the individual scattering processes show that the dominant scattering mechanisms are the alloy disorder scattering and the interface roughness scattering at low temperatures. The variation of 2DEG mobility with the barrier layer parameters results mainly from the change of 2DEG density and distribution. It is suggested that in AlGaN/GaN samples with a high Al content or a thick AlGaN layer, the interface roughness scattering may restrict the 2DEG mobility significantly, for the AlGaN/GaN interface roughness increases due to the stress accumulation in AlGaN layer.

Low-thermal-budget Au-free ohmic contact to an ultrathin barrier AlGaN/GaN heterostructure utilizing a micro-patterned ohmic recess

A pre-ohmic micro-patterned recess process,is utilized to fabricate Ti/Al/Ti/TiN ohmic contact to an ultrathin-barrier(UTB)AlGaN/GaN heterostructure,featuring a significantly reduced ohmic contact resistivity of 0.56Ω·mm at an alloy temperature of 550℃.The sheet resistances increase with the temperature following a power law with the index of +2.58,while the specific contact resistivity decreases with the temperature.The contact mechanism can be well described by thermionic field emission(TFE).The extracted Schottky barrier height and electron concentration are 0.31 eV and 5.52×10^(18) cm^(−3),which suggests an intimate contact between ohmic metal and the UTB-AlGaN as well as GaN buffer.A good correlation between ohmic transfer length and the micro-pattern size is revealed,though in-depth investigation is needed.A preliminary CMOS-process-compatible metal-insulator-semiconductor high-mobility transistor(MIS-HEMT)was fabricated with the proposed Au-free ohmic contact technique.

Influence of thermal stress on the characteristic parameters of AlGaN/GaN heterostructure Schottky contacts

Ni Schottky contacts on A1GaN/GaN heterostructures have been fabricated. The samples are then thermally treated in a furnace with N2 ambient at 600℃ for different times （0.5, 4.5, 10.5, 18, 33, 48 and 72 h）. Current-voltage （I-V） and capacitance-voltage （C-V） relationships are measured, and SchrSdinger＇s and Poisson＇s equations are self- consistently solved to obtain the characteristic parameters related to A1GaN/GaN heterostructure $chottky contacts： the two-dimensional electron gas （2DEG） sheet density, the polarization sheet charge density, the 2DEG distribution in the triangle quantum well and the Schottky barrier height for each thermal stressing time. Most of the above parameters reduce with the increase of stressing time, only the parameter of the average distance of the 2DEG from the A1CaN/GaN interface increases with the increase of thermal stressing time. The changes of the characteristic parameters can be divided into two stages. In the first stage the strain in the A1GaN barrier layer is present. In this stage the characteristic parameters change rapidly compared with those in the second stage in which the AlGaN barrier layer is relaxed and no strain is present.

Abnormal phenomenon of source-drain current of AlGaN/GaN heterostructure device under UV/visible light irradiation

We report an abnormal phenomenon that the source-drain current(I_(D))of AlGaN/GaN heterostructure devices decreases under visible light irradiation.When the incident light wavelength is 390 nm,the photon energy is less than the band gaps of GaN and AlGaN whereas it can causes an increase of ID.Based on the UV light irradiation,a decrease of I_(D) can still be observed when turning on the visible light.We speculate that this abnormal phenomenon is related to the surface barrier height,the unionized donor-like surface states below the surface Fermi level and the ionized donor-like surface states above the surface Fermi level.For visible light,its photon energy is less than the surface barrier height of the AlGaN layer.The electrons bound in the donor-like surface states below the Fermi level are excited and trapped by the ionized donor-like surface states between the Fermi level and the conduction band of AlGaN.The electrons trapped in ionized donor-like surface states show a long relaxation time,and the newly ionized donor-like surface states below the surface Fermi level are filled with electrons from the two-dimensional electron gas(2DEG)channel at AlGaN/GaN interface,which causes the decrease of ID.For the UV light,when its photon energy is larger than the surface barrier height of the AlGaN layer,electrons in the donor-like surface states below the Fermi level are excited to the conduction band and then drift into the 2DEG channel quickly,which cause the increase of ID.

The effects of vicinal sapphire substrates on the properties of AlGaN/GaN heterostructures

A1GaN/GaN heterostructures on vicinal sapphire substrates and just-oriented sapphire substrates （0001） are grown by the metalorganic chemical vapor deposition method. Samples are studied by high-resolution x-ray diffraction, atomic force microscopy, capacitance-voltage measurement and the Van der Panw Hall-effect technique. The investigation reveals that better crystal quality and surface morphology of the sample are obtained on the vicinal substrate. Fur- thermore, the electrical properties are also improved when the sample is grown on the vicinal substrate. This is due to the fact that the use of vicinal substrate can promote the step-flow mode of crystal growth, so many macro-steps are formed during crystal growth, which causes a reduction of threading dislocations in the crystal and an improvement in the electrical properties of the AlGaN/GaN heterostructure.

Determination of the relative permittivity of the AlGaN barrier layer in strained AlGaN/GaN heterostructures

Using the measured capacitance voltage curves and the photocurrent spectrum obtained from the Ni Schottky contact on a strained Al0.3Ga0.7N/GaN heterostructure, the value of the relative permittivity of the AlGaN barrier layer was analysed and calculated by self-consistently solving SchrSdinger＇s and Poisson＇s equations. It is shown that the calculated values of the relative permittivity are different from those formerly reported, and reverse biasing the Ni Schottky contact has an influence on the value of the relative permittivity. As the reverse bias increases from 0 V to -3 V, the value of the relative permittivity decreases from 7.184 to 7.093.

Degradation mechanism of two-dimensional electron gas density in high Al-content AlGaN/GaN heterostructures

This paper finds that the two-dimensional electron gas density in high Al-content A1GaN/GaN heterostructures exhibits an obvious time-dependent degradation after the epitaxial growth. The degradation mechanism was investigated in depth using Hall effect measurements,high resolution x-ray diffraction,scanning electron microscopy,x-ray photoelectron spectroscopy and energy dispersive x-ray spectroscopy.The results reveal that the formation of surface oxide is the main reason for the degradation,and the surface oxidation always occurs within the surface hexagonal defects for high Al-content AlGaN/GaN heterostructures.

Self-aligned-gate AlGaN/GaN heterostructure field-effect transistor with titanium nitride gate

Self-aligned-gate heterostructure field-effect transistor（HFET） is fabricated using a wet-etching method.Titanium nitride（TiN） is one kind of thermal stable material which can be used as the gate electrode.A Ti/Au cap layer is fixed on the gate and acts as an etching mask.Then the T-shaped gate is automatically formed through over-etching the TiN layer in 30% H2O2 solution at 95 ℃.After treating the ohmic region with an inductively coupled plasma（ICP） method,an Al layer is sputtered as an ohmic electrode.The ohmic contact resistance is approximately 0.3 Ω·mm after annealing at a low-temperature of 575 ℃ in N2 ambient for 1 min.The TiN gate leakage current is only 10^-8 A after the low-temperature ohmic process.The access region length of the self-aligned-gate（SAG） HFET was reduced from 2 μm to 0.3 μm compared with that of the gate-first HFET.The output current density and transconductance of the device which has the same gate length and width are also increased.

Analysis of the abnormal resistance in AlGaN/GaN heterostructure's ohmic contact tests

Ti/Al/Ti/Au and Ti/Al/Ni/Au ohmic contacts were fabricated on AlGaN/GaN heterostructure under different temperatures of rapid thermal processing （RTP）. Since abnormal resistance values were observed during the contact resistance testing,the surface morphology and contact borders of the samples were analyzed to determine the physical mechanism. Such abnormal phenomenon is found to originate from cracking of the AlGaN layer during RTP,flowing of Ti/Al metallic liquid along the crevices,and continuous reaction of the metallic liquid with AlGaN/GaN. Such processes result in abnormal conduction channels. The possible mechanism of the crevice formation was discussed,and the possible solutions to avoid the crevices were proposed.

Influence of surface states on deep level transient spectroscopy in AlGaN/GaN heterostructure

Deep level transient spectroscopy（DLTS） as a method to investigate deep traps in AlGaN/GaN heterostructure or high electron mobility transistors（HEMTs） has been widely utilized.The DLTS measurements under different bias conditions are carried out in this paper.Two hole-like traps with active energies of E_v ＋ 0.47 eV,and E_v ＋ 0.10 eV are observed,which are related to surface states.The electron traps with active energies of E_c-0.56 eV are located in the channel,those with E_c-0.33 eV and E_c-0.88 eV are located in the AlGaN layer.The presence of surface states has a strong influence on the detection of electron traps,especially when the electron traps are low in density.The DLTS signal peak height of the electron trap is reduced and even disappears due to the presence of plentiful surface state.

Synthesis of thermally stable HfO_xN_y as gate dielectric for AlGaN/GaN heterostructure field-effect transistors

In this paper, we adopted thermally stable HfOxNy as gate dielectric for TiN/HfOxNy/A1GaN/GaN heterostructure field-effect transistors （HFETs） application. It demonstrated that the surface morphologies, composition, and optical properties of the HfOxNy films were dependent on oxygen flow rate in the O2/N2/mr mixture sputtering ambient. The obtained metal-oxide-semiconductor heterostructure field-effect transistors by depositing HfO2 and HfOxNy dielectric at different oxygen flow rates possessed a small hysteresis and a low leakage current. After post deposition annealing at 900 ℃, the device using HfOxNy dielectric operated normally with good pinch-off characteristics, while obvious degradation are observed for the HfO2 gated one at 600 ℃. This result shows that the HfOxNy dielectric is a promising candidate for the self-aligned gate process.

Effects of post-annealed floating gate on the performance of AlGaN/GaN heterostructure field-effect transistors

AlGaN/GaN heterostructure field-effect transistors （HFETs） with different floating gate lengths and floating gates annealed at different temperatures, are fabricated. Using the measured capacitance-voltage curves of the gate Shottky contacts for the AlGaN/GaN HFETs, we find that after floating gate experiences 600℃ rapid thermal annealing, the larger the floating gate length, the larger the two-dimensional electron gas electron density under the gate region is. Based on the measured capacitance-voltage and current-voltage curves, the strain of the AlGaN barrier layer in the gate region is calculated, which proves that the increased electron density originates from the increased strain of the AlGaN barrier layer.

Groove-type channel enhancement-mode Al GaN/GaN MIS HEMT with combined polar and nonpolar AlGaN/GaN heterostructures

A novel groove-type channel enhancement-mode AlGaN/GaN MIS high electron mobility transistor（GTCE-HEMT）with a combined polar and nonpolar AlGaN/GaN heterostucture is presented. The device simulation shows a threshold voltage of 1.24 V, peak transconductance of 182 m S/mm, and subthreshold slope of 85 m V/dec, which are obtained by adjusting the device parameters. Interestingly, it is possible to control the threshold voltage accurately without precisely controlling the etching depth in fabrication by adopting this structure. Besides, the breakdown voltage（VB） is significantly increased by 78% in comparison with the value of the conventional MIS-HEMT. Moreover, the fabrication process of the novel device is entirely compatible with that of the conventional depletion-mode（D-mode） polar AlGaN/GaN HEMT. It presents a promising way to realize the switch application and the E/D-mode logic circuits.

Plasma-assisted surface treatment for low-temperature annealed ohmic contact on AlGaN/GaN heterostructure field-effect transistors

In this study, a low-temperature annealed ohmic contact process was proposed on AlGaN/GaN heterostructure field effect transistors （HFETs） with the assistance of inductively coupled plasma （ICP） surface treatment. The effect of ICP treatment process on the 2DEG channel as well as the formation mechanism of the low annealing temperature ohmic contact was investigated. An appropriate residual AlGaN thickness and a plasma-induced damage are considered to contribute to the low-temperature annealed ohmic contact. By using a single Al layer to replace the conventional Ti/Al stacks, ohmic contact with a contact resistance of 0.35 Ω.mm was obtained when annealed at 575 ℃ for 3 min. Good ohmic contact was also obtained by annealing at 500 ℃ for 20 rain.

Si and Mg pair-doped interlayers for improving performance of AlGaN/GaN heterostructure field effect transistors grown on Si substrate

We report a novel structure of A1GaN/GaN heterostructure field effect transistors （HFETs） with a Si and Mg pair- doped interlayer grown on Si substrate. By optimizing the doping concentrations of the pair-doped interlayers, the mobility of 2DEG increases by twice for the conventional structure under 5 K due to the improved crystalline quality of the conduction channel. The proposed HFET shows a four orders lower off-state leakage current, resulting in a much higher on/off ratio （ - 10^9）. Further temperature-dependent performance of Schottky diodes revealed that the inhibition of shallow surface traps in proposed HFETs should be the main reason for the suppression of leakage current.

Influence of thermal stress on the relative permittivity of the AlGaN barrier layer in an AlGaN/GaN heterostructure Schottky contacts

Ni Schottky contacts on A1GaN/CaN heterostructures were fabricated. Some samples were thermally treated in a furnace with N2 ambience at 600 ~C for different times （0.5 h, 4.5 h, 10.5 h, 18 h, 33 h, 48 h, and 72 h）, the others were thermally treated for 0.5 h at different temperatures （500 ~C, 600 ~C, 700 ~C, and 800 ~C）. With the measured current-voltage （I-V） and capacitance-voltage （C V） curves and by self-consistently solving Schrodinger＇s and Poisson＇s equations, we found that the relative permittivity of the A1GaN barrier layer was related to the piezoelectric and the spontaneous polarization of the A1GaN barrier layer. The relative permittivity was in proportion to the strain of the A1GaN barrier layer. The relative permittivity and the strain reduced with the increased thermal stress time until the A1GaN barrier totally relaxed （after 18 h at 600 ~C in the current study）, and then the relative permittivity was almost a constant with the increased thermal strcss time. When the sample was treated at 800 ~C for 0.5 h, the relative permittivity was less than the constant due to the huge diffusion of the contact metal atoms. Considering the relation between the relative permittivity of the A1GaN barrier layer and the converse piezoelectric effect, the conclusion can be made that a moderate thermal stress can restrain the converse piezoelectric effect and can improve the stability of A1GaN/GaN heterostructure devices.

A simple method of extracting the polarization charge density in the AlGaN/GaN heterostructure from current-voltage and capacitance-voltage characteristics

An Ni Schottky contact on the A1GaN/GaN heterostructure is fabricated. The flat-band voltage for the Schottky contact on the A1GaN/GaN heterostructure is obtained from the forward current-voltage characteristics. With the measured capacitance-voltage curve and the flat-band voltage, the polarization charge density in the A1GaN/GaN heterostructure is investigated, and a simple formula for calculating the polarization charge density is obtained and analyzed. With the approach described in this paper, the obtained polarization charge density agrees well with the one calculated by self-consistently solving Schrodinger＇s and Poisson＇s equations.

Preparation of AlGaN/GaN Heterostructures on Sapphire Using Light Radiation Heating Metal-Organic Chemical Vapor Deposition at Low Pressure

AlGaN/GaN heterostructures on sapphire substrate were fabricated by using light radiation heating metalor ganic chemical vapor deposition.Photoluminescence excitation spectra show that there are two abrupt slopes corresponding to the absorption edges of AlGaN and GaN,respectively.X-ray diffraction spectra clearly exhibit the GaN(0002),(0004),and A1GaN(0002),(0004)diffraction peaks,and no diffraction peak other than those from the GaN{0001}and A1GaN{0001}planes is found.Reciprocal space mapping indicates that there is no tilt between the AlGaN layer and the GaN layer.All results also indicate that the sample is of sound quality and the Al composition in the AlGaN layer is of high uniformity.

Distribution of donor states on the surface of AlGaN/GaN heterostructures

The uniform distribution model of the surface donor states in AlGaN/GaN heterostructures has been widely used in the theoretical calculation.A common and a triple-channel AlGaN/GaN heterostructure Schottky barrier diodes have been fabricated to verify the models,but the calculation results show the uniform distribution model can not provide enough electrons to form three separate 2DEGs in the triple-channel Al GaN/GaN heterostructure.Our experiments indicate the uniform distribution model is not quite right,especially for the multiple-channel AlGaN/GaN heterostructures.Besides,it is found the exponential distribution model possibly matches the actual distribution of the surface donor states better,which allows the 2DEG to form in each channel structure during the calculation.The exponential distribution model would be helpful in the research field.

Recess-free enhancement-mode AlGaN/GaN RF HEMTs on Si substrate

Enhancement-mode(E-mode)GaN-on-Si radio-frequency(RF)high-electron-mobility transistors(HEMTs)were fabri-cated on an ultrathin-barrier(UTB)AlGaN(<6 nm)/GaN heterostructure featuring a naturally depleted 2-D electron gas(2DEG)channel.The fabricated E-mode HEMTs exhibit a relatively high threshold voltage(VTH)of+1.1 V with good uniformity.A maxi-mum current/power gain cut-off frequency(fT/fMAX)of 31.3/99.6 GHz with a power added efficiency(PAE)of 52.47%and an out-put power density(Pout)of 1.0 W/mm at 3.5 GHz were achieved on the fabricated E-mode HEMTs with 1-μm gate and Au-free ohmic contact.