CN1140930C - AlxGa1-xN/GaN Heterojunction Ferroelectric/Semiconductor Memory Fabrication Method - Google Patents

Abstract

Based on Al _x Ga _1-x N/GaN heterojunction ferroelectric/semiconductor memory structure and its manufacturing method, Al _x Ga _1-x N/GaN modulation doping was first grown on sapphire substrate by MOCVD technology Heterostructure, then grow PZT ferroelectric film on AlxGa _1-x N by PLD technology, finally use electron beam evaporation technology to deposit Ti/Al ohmic contact electrode on _{AlxGa 1-x} N layer and PZT layer Al electrode is deposited on it. This structure takes advantage of the high-temperature stability of the PZT/ _{AlxGa1 -xN} interface and avoids the interfacial interdiffusion and interfacial reaction problems of common ferroelectric/Si MFS structures. At the same time, this structure uses the high-concentration and high-mobility two-dimensional electron gas at the _{AlxGa1 -xN} /GaN heterointerface as channel carriers, which is conducive to improving the response speed of the memory structure and other properties.

Description

Fabrication Method of AlxGa1-xN/GaN Heterojunction Ferroelectric/Semiconductor Memory

1. Technical field

The present invention relates to a ferroelectric/semiconductor memory structure based on _{AlxGa1 -xN} /GaN heterojunction, including ferroelectric polarization effect in ferroelectric and _{AlxGa1 -xN} layer The modulation mechanism of the piezoelectric polarization effect on the two-dimensional electron gas density at the Al _x Ga _1-x N/GaN heterointerface, and the preparation method of this memory structure.

2. Technical Background

Since the 1970s, various efforts have been made internationally to introduce ferroelectrics into microelectronics technology based on semiconductor materials by using the extremely strong polarization effect and high relative permittivity of ferroelectric materials. Among them, the most promising device structure is metal-ferroelectric-semiconductor field-effect transistor (MFS-FET), which can be used to make non-volatile read-only memory.

From the perspective of semiconductor devices, MFS-FET still belongs to the category of metal-insulator-semiconductor field effect transistor (MIS-FET), but in the MFS-FET structure, ferroelectrics are used as insulators instead of general metal-oxide - _SiO2 in semiconductor (MOS) field effect devices. For a long time, people have been using Si material as the semiconductor channel material in MFS-FET. The main advantage is that it is compatible with the existing semiconductor MOS device technology in device preparation. However, the main technical problem of this structure is that the deposition of ferroelectric film on Si must be carried out at high temperature, and the ferroelectric film needs to be subjected to thermal annealing after deposition. During these processes, the ferroelectric/Si interface The interdiffusion of atoms is very serious, and interfacial solid-state reaction occurs, so the properties of the ferroelectric/Si interface are difficult to control. At the same time, the high interface state density existing at the ferroelectric/Si interface seriously destroys the characteristics of the MFS-FET memory structure. These problems have seriously restricted the development of Si-based ferroelectric memory for many years.

Group III nitride wide bandgap semiconductor materials (including GaN, AlN, InN and their ternary alloys) are the third-generation new semiconductor materials that have been highly valued internationally in recent years. They have high temperature resistance, corrosion resistance, high saturation electron drift velocity, and high Excellent physical and chemical properties such as breakdown field strength and direct band gap. Al _x Ga _1-x N/GaN heterostructure is considered to be the preferred material system for the development of high-temperature, high-power, and high-frequency semiconductor devices, and Al _x Ga _1-x N/GaN heterostructure field-effect transistors (HFETs), and It is said that the development level of high electron mobility transistor (HEMT) has been rapidly improved, the process technology is basically mature, and the device performance is close to practical use. At the same time, the research on _{AlxGa1 -xN} /GaN MIS-HFET with _SiO2 as the gate material has also received great attention. Therefore, if the III-nitride material, especially the _{AlxGa1 -xN} /GaN heterostructure material is used to replace the Si material for the development of the MFS-HFET memory structure, the high temperature of the ferroelectric/Si interface can be solved. Instability problems can also make full use of the good transport properties of the _{AlxGa1 -xN} /GaN heterointerface two-dimensional electron gas (2DEG) to improve the response speed of this type of memory structure.

3. Contents of the invention

The object of the present invention is to develop the MFS structure with _{AlxGa1 -xN} /GaN heterostructure as the semiconductor channel, realize its storage performance, and improve the response speed of this type of memory structure.

The purpose of the present invention is achieved like this:

Using sapphire as the substrate, there is a high-quality Al _x Ga _1-x N/GaN modulation doped heterostructure, and the value of X is between 0.15-0.30; then grow PZT on the Al _x Ga _1-x N layer For the ferroelectric thin film, the bottom electrode and the top electrode are respectively prepared on the Al _x Ga _1-x N layer and the PZT layer.

The GaN thickness is 1-2um, the _{AlxGa1 -xN} layer thickness is 10-100nm, and the PZT layer thickness is 100-500nm.

Due to the lattice mismatch between _{AlxGa1 -xN} and GaN, and the high piezoelectric coefficient of _{AlxGa1 -xN} , there is a strong piezoelectricity in the _{AlxGa1 -xN} layer on GaN The polarization effect leads to the formation of a two-dimensional electron gas (2DEG) with a concentration of ~10 ¹³ cm ^-2 at the heterogeneous interface of Al _x Ga _1-x N/GaN, and the mobility of 2DEG reaches more than 1000 cm ² /Vs, thereby forming Al _x An ideal channel in Ga _1-x N/GaN based MFS structure.

A Pb(Zr _0.53 Ti _0.47 )O ₃ (lead zirconate titanate, PZT for short) film is deposited on the Al _x Ga _1-x N layer by pulsed laser deposition (PLD) technology. PZT is a typical ferroelectric material, its residual polarization charge can reach 10μC/cm ² under zero electric field, and its relative permittivity can be as high as 1000 or more. A very thin PZT film (hundreds of nanometers) can generate a strong polarization electric field to modulate the 2DEG concentration in the _{AlxGa1 -xN} /GaN heterointerface channel.

Finally, the Al electrode (top electrode) is made on the PZT film by the electron beam evaporation method, and the Ti/Al ohmic contact electrode (bottom electrode) is made on the Al _x Ga _1-x N/GaN to form an electrical property measurement. _{AlxGa1 -xN} /GaN based MFS structure.

The present invention adopts the Al _x Ga _1-x N/GaN heterostructure as the semiconductor channel of the MFS memory structure for the first time in the world. The main innovations include: (1) Due to the high temperature stability of the III-nitride material, Solve the problems of high-temperature instability and interfacial solid-state reaction of the ferroelectric/Si interface; (2) Utilize the strong polarization effect of the Al _x Ga _1-x N layer, on the one hand make the Al _x Ga _1-x N/ The GaN heterointerface produces a two-dimensional electron gas with high concentration and high mobility, forming an ideal device channel; on the other hand, the polarization field formed by the _{AlxGa1 -xN} layer and the polarization field formed by the PZT layer The fields (in opposite directions under negative bias) act together to make the capacitance-voltage (CV) storage window of the MFS structure fully realized under negative bias. This means that the CV storage window can be created without the need for polarization inversion of the PZT ferroelectric film, thus greatly reducing various problems caused by the ferroelectric inversion fatigue effect in the Si-based MFS structure. This feature of realizing CV storage window without ferroelectric polarization inversion is impossible for Si-based MFS structure.

4. Description of drawings

Figure 1: Schematic diagram of PZT/Al _0.22 Ga _0.78 N/GaN MFS structure

Figure 2: High-resolution X-ray diffraction ω/2θ rocking curves of Al _0.22 Ga _0.78 N/GaN modulated doped heterostructures. Multiple satellite peaks illustrate the high quality of the heterostructure and the steepness of the heterointerface. This is the basis for the formation of high-concentration and high-mobility 2DEG at the Al _0.22 Ga _0.78 N/GaN heterointerface.

Figure 3: (a) Schematic diagram of the PZT/Al _0.22 Ga _0.78 N/GaN MFS structure, (b) schematic diagram of the charge distribution of the structure under negative bias, (c) schematic diagram of the conduction band structure, the solid line indicates the presence of PZT iron electrodes The dotted line represents the case of no PZT ferroelectric polarization, P _f represents the ferroelectric polarization vector in the PZT layer (under negative bias), P _p represents the piezoelectric polarization vector in the Al _0.22 Ga _0.78 N layer, P _f is opposite to P _p . P _f varies with the applied bias voltage. Under negative bias, it raises the conduction band bottom of the GaN layer and makes the Al _0.22 Ga _0.78 N/GaN heterointerface triangular quantum well shallower, resulting in a decrease in the 2DEG concentration. P _p does not change with the applied bias voltage, and its effect is just opposite to that of P _f , making the Al _0.22 Ga _0.78 N/GaN interface quantum well deeper, resulting in an increase in the 2DEG concentration.

Figure 4: High-frequency CV curve of Al _0.22 Ga _0.78 N/GaN-based MFS structure at 1MHz. The whole picture is the curve of the entire voltage scanning range, and the inner picture is the curve under negative bias. When the bias voltage is greater than 0.7V (forward bias), the capacitance C depends on the PZT film. Due to its large relative permittivity, C is very large; when the bias voltage becomes negative bias, the voltage is added to Al _0.22 Ga _0.78 N On the upper layer, C drops sharply. The change in C around -9V bias is due to the depletion of 2DEG.

Figure 5: The CV scanning hysteresis loop of the Al _0.22 Ga _0.78 N/GaN-based MFS structure, in the counterclockwise direction. The CV ferroelectric storage window is around -9V with a width of 0.2V, which is caused by the different ferroelectric polarization states during the voltage forward scan and reverse scan, and the entire CV storage window is in the negative bias range, indicating that the PZT film The ferroelectric polarization does not reverse.

5. Specific implementation methods

The Al _0.22 Ga _0.78 N/GaN modulation-doped heterostructure was grown by MOCVD on the sapphire with (0001) plane as the substrate. When growing, use GaN as a buffer layer (thickness 30nm) at a growth temperature of 488°C; then, epitaxially grow a GaN layer (thickness 2μm) at a growth temperature of 1030°C; then grow an undoped Al _0.22 Ga _0.78 N layer with a thickness of 3nm and grow The temperature is 1080°C; finally grow a Si-doped n-type Al _0.22 Ga _0.78 N layer with a thickness of 75nm and a growth temperature of 1080°C. The MOCVD growth is carried out under normal pressure, the growth sources are trimethylgallium (TMG), trimethylaluminum (TMA) and high-purity ammonia (NH ₃ ), and the carrier gas and dilution gas are hydrogen (H ₂ ). The Al _0.22 Ga _0.78 N layer composition ratio is determined by the flow ratio of TMG and TMA.

On the Al _0.22 Ga _0.78 N/GaN heterostructure, pulsed laser deposition (PLD) is used to grow PZT thin films with a film thickness of 400nm. The laser is a KrF excimer laser (wavelength 248nm), and 2.5J/cm2 is formed on the target surface during deposition. The energy density of ² , the growth temperature is 750°C.

Finally, the Al electrode (top electrode) is made on the PZT film by electron beam evaporation, and the Ti/Al ohmic contact electrode (bottom electrode) is made on the Al _x Ga _1-x N/GaN to form Al _0.22 Ga _0.78 N/ GaN-based MFS structure.

The preparation of high-quality Al _x Ga _1-x N/GaN modulation-doped heterostructure is the core technology for the preparation of Al _x Ga _1-x N/GaN-based MFS storage structures. The following process is a typical Al _0.22 Ga _0.78 N/ GaN modulation doped heterostructure MOCVD growth process:

Growth and annealing of GaN buffer layer:

TMG flow: 15μmol/min, NH ₃ flow: 3.5SLM/min, H ₂ flow: 3.0SLM/min

Growth temperature: 488°C

Growth time: 140 seconds, thickness 30nm

Growth Pressure: 760 Torr

Annealing after growth: H ₂ flow: 1.0SLM/min, NH ₃ flow: 0.5SLM/min; 1030°C; 5min

Non-doped GaN (i-GaN) epitaxial layer growth:

TMG flow: 60μmol/min, NH ₃ flow: 4.0SLM/min, H ₂ flow: 0.5SLM/min

Growth temperature: 1030°C

Growth time: 60min, thickness 2μm

Growth pressure: 760Torr

Growth of non-doped Al _0.22 Ga _0.78 N (i-AlGaN) epitaxial layer:

TMG flow: 10μmol/min, TMA flow: 12μmol/min, NH ₃ flow: 4.0SLM/min,

H ₂ flow rate: 0.5SLM/min

Growth temperature: 1080°C

Growth time: 39 seconds, thickness 3nm

Growth pressure: 760Torr

Si-doped n-type Al _0.22 Ga _0.78 N (n-AlGaN) epitaxial layer growth:

TMG flow: 10μmol/min, TMA flow: 12μmol/min, NH ₃ flow: 4.0SLM/min,

H ₂ flow: 0.5SLM/min, SiH ₄ flow: 1.0sccm/min

Growth temperature: 1080°C

Growth time: 675 seconds, thickness 75nm

Growth pressure; 760Torr

Claims (1)

1.Al _xGa _1-xThe method for making of the metal/ferroelectric of N/GaN heterojunction/semiconductor (MFS) memory structure, it is characterized in that the sapphire with (0001) face is a substrate, after routine is cleaned, adopt the MOCVD growing technology, through preliminary treatment, GaN buffer growth and annealing, non-Doped GaN outer layer growth, non-doped with Al _xGa _1-xN outer layer growth and Si Doped n-type Al _xGa _1-xN outer layer growth several stages is finished Al _xGa _1-xThe preparation of N/GaN modulation-doped heterostructure, growth source are respectively trimethyl gallium (TMG), trimethyl aluminium (TMA) and high-purity ammonia (NH ₃), carrier gas and diluent gas are hydrogen (H ₂); Again at Al _xGa _1-xGrowth PZT ferroelectric thin film on the N layer, the PZT ferroelectric thin film adopts the preparation of pulsed laser deposition (PLD) technology; Adopt electron beam evaporation respectively at Al _xGa _1-xDeposit Ti/Al Ohm contact electrode is as hearth electrode on the N layer, and deposit Al electrode is as top electrode on the PZT layer.

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