JPS6337676A - Photoconductive device - Google Patents

Abstract

PURPOSE:To facilitate a high speed response by a method wherein one layer of photoconductive p<+> (or n<+>) type semiconductor thin film is inserted between two layers of photoconductive n<-> (or p<->) type semiconductor thin films to constitute a multilayered film structure. CONSTITUTION:A multilayered film is composed of n<-> (or p<->) type semiconductor thin films 5 and p<+> (or n<+>) type semiconductor thin films 6 and formed by accumulating the n<-> (or p<->) type semiconductor thin films 5 and the p<+> (or n<+>) type semiconductor thin films 6 alternately and successively on a substrate. Then 1st electrode 2 and 2nd electrode 3 are formed by vacuum evaporation on one pair of facing wall surfaces of the multilayered film so as to be contacted with all the n<-> (or p<->) type semiconductor thin films 5. In the same way, 3rd electrode 4 is formed on one wall surface of the other pair of facing wall surfaces of the multilayered film so as to be contacted with all the p<+> (or n<+>) type semiconductor thin films 6. The 1st electrode 2 and the 2nd electrode 3 formed like this are connected to the n<-> (or p<->) type semiconductor thin films 5 and the 3rd electrode 4 formed like this is connected to the p<+> (or n<+>) type semiconductor thin films 6 electrically. Then ohmic regions 7 are formed.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to a photoconductive element for use in ultra-high-speed electronic measurement, and in particular, to a photoconductive element for use in ultra-high-speed electronic measurement, and in particular to a high-speed response by shortening the lifetime of minority carriers in a semiconductor. and a semiconductor device I'i whose purpose is to improve the sensitivity characteristics to the wavelength of light.
The present invention relates to photoconductive elements belonging to the field of microelectronics, which are characterized by a thin film structure.

Recently, by making full use of ultra-thin film manufacturing technology and microfabrication technology,
E M T (1 layergh Electron Mob
ity Transist-or) and HBT
(lettero Bipolar Transist
Ultrahigh-speed devices such as or) are rapidly being put into practical use and are being used as key devices in various electronic devices.

In the prior art, measurement using a purely electrical sampling oscilloscope is known as a means of waveform analysis and speed evaluation of high-speed elements.

However, with this device, the time resolution is at most 20 ps.
e(, therefore, it has been difficult to observe and measure waveforms of ultrahigh-speed devices such as HEMTs that exhibit rise characteristics on the order of 10 psec.

Therefore, focusing on the high speed of light, research has recently been conducted to solve this problem by using optical sampler techniques, that is, switching electrical signals with light or detecting the electrical signals.

The photoconductive element of the present invention is being considered for various applications such as waveform observation of ultra-high-speed elements such as HEMTs and HBTs, as well as optical gate elements and signal generators. It is highly desired in the field of high-speed electronic measurement.

<Conventional technology> Research on applying photoconductive elements to the field of ultrahigh-speed electronic measurement is C.H. of the University of Maryland (USA) in 1972;

It began with Lee's confirmation that the photoconductive effect of semiconductors has a high-speed response.

Regarding this, see the document Picosecond 0pto
electr-onic devices C,ll,
Lee+ ＾cademic Press+ New
It is introduced in detail in York (1984).

However, subsequent research has shown that the cause of inhibiting the high-speed response of devices is due to excited minority carriers and the long lifetime of trap levels, which are unevenly distributed in the forbidden band especially in compound semiconductors. In order to solve these problems, there are methods to forcibly introduce recombination centers into the forbidden band of semiconductors using ion implantation technology, increasing the trapping cross section for carriers, and methods using liquid phase crystal growth. A photoelectric material with a different conductivity type is formed on the substrate, that is, p
A method has been adopted to eliminate the minority carrier by creating a -n junction structure and applying a reverse bias between the pn junctions and utilizing a high electric field in the depletion layer.

Next, looking at the dependence of the wavelength sensitivity of photoconductive elements on the light used, conventional photoconductive elements made of a single material have light absorption characteristics and photoconductive effects that depend on the wavelength of the light used. It was found that the wavelength sensitivity characteristics were not uniform depending on the light.

<Problems to be solved by the invention> The method of shortening the lifetime of minority carriers and traps using ion implantation is to form crystal defects in the semiconductor, which leads to a decrease in mobility and property deterioration. A problem arises.

In addition, when a p-n junction is formed by liquid phase growth, etc., the film thickness of the photoelectric material becomes thick, and the mean free path of minority carriers is limited, so carriers are naturally absorbed by the substrate side. As a result, only unsatisfactory results have been obtained in terms of high-speed response of the device.

In consideration of the above, the present invention has excellent characteristics that can be used in the field of high-speed electronic measurement, that is, high sensitivity, excellent response characteristics (high-speed response), and no characteristic deterioration. The purpose of this invention is to realize a photoconductive element that has uniform wavelength sensitivity characteristics for the light used.

Therefore, in the present invention, in order to realize the above-mentioned photoconductive element, it is possible to manufacture an ultra-thin film, and moreover, two or more semiconductor materials are epitaxially crystallized alternately or simultaneously. M B E (Molecular B
eam [1pitaxy) technology is used.

In order to achieve high-speed response, which is the first objective of the present invention, the thickness of the n (or p-) type semiconductor thin film having two or more layers of photoconductivity should not exceed the range of at most 300 nm. One layer of p (or n
) type semiconductor V# film is inserted to create a multilayer film structure.

In addition, in order to improve the wavelength sensitivity of light, which is the second objective, we used a semiconductor made of a mixed crystal of two types of substances (a first substance and a second substance), and In this method, a multilayer film is formed in which the mixed crystal ratio is changed so that the sensitivity gradually changes at short wavelengths.

Effect> The photoconductive element of the present invention manufactured in this manner has a lower defect density than conventional elements in which recombination centers are forcibly introduced, and therefore increases dark conductivity and reduces mobility. It is possible to minimize the adverse effects on the device characteristics, such as a decrease in the characteristics of the device.

In addition, since it has a multilayer film structure, a p-n junction depletion layer is formed between each layer in the multilayer film, so the high electric field in the depletion layer is utilized to transfer minority carriers with high probability to p (or n ) side and remove it.

Short wavelength light has a larger light absorption coefficient in a semiconductor than long wavelength light.

Therefore, in the multilayer film, the ratio of the mixed crystals of the two types of semiconductors can be changed so that the light incident side becomes highly sensitive to shorter wavelengths.

In this way, the wavelength sensitivity of the multilayer film to light can be made constant.

<Example> Hereinafter, one embodiment of the present invention will be described with reference to the drawings.

FIG. 1 shows an example of the structural concept and method of forming the photoconductive element of the present invention having a multilayer film structure.

As shown in the figure, in the present invention, a substrate 1 has an n (or p) type semiconductor thin film 5 having high mobility on it as an epitaxial single crystal (layer), so that
A material whose lattice constant is equal to or very close to that of this semiconductor thin film is used.

Usually, the same semiconductor material is used.

This substrate also has an n- (or p-) type semiconductor thin film (
5) has a conductivity type opposite to 5) and has a low specific resistance (ρ
<10Ωcm).

Then, a multilayer film is formed on the substrate.

This multilayer film consists of an n-(or p″″) semiconductor thin film 5 and a p(
or n) type semiconductor thin film I5!6, and the n- (or p) type semiconductor thin film 5 and the p'' (or n) type semiconductor thin film 6 are alternately and sequentially formed on the substrate by f(iffl). Form.

For this formation, M
The BE method was used in this example.

Next, in this example, the first electrode 2 and the second electrode 3 are
A layer is formed on one opposing wall surface of the multilayer film using a vacuum evaporation method so as to contact all of the n- (or p-> type semiconductor thin film 5).

Similarly, a third electrode 4 is formed on one side of the other opposing wall surface of the multi-day film so as to be in contact with the substrate 1 and all of the p (or n+) type semiconductor thin film 6.

In this case, the bridge material is, for example, an Au-Ge-Ni alloy for a GaAs-based compound semiconductor, an Au-3n alloy for an InP-based compound semiconductor, and an Au-3n alloy for a silicon-germanium mixed crystal semiconductor. Au-Ni alloy or the like is used.

The first electrode 2 and the second electrode 3 thus formed are formed with an n (or p-) type semiconductor thin film 5, and the third electrode 4 is formed with a p'' (or n) type semiconductor thin film 6. It will be electrically connected.

Then, the ohmic region 7 is formed by an alloy method. That is, by performing a short heat treatment at around 400°C and alloying the semiconductor and the electric +IiA metal, the interface lS
@ Reduce the wall.

Here, between the first electrode 2 and the p (or n) type semiconductor thin film 6, between the second electrode 3 and the p (or n) type semiconductor thin film 6, and between the third electrode 4 and the n- (or p) type It is necessary to electrically insulate the semiconductor thin films 5 from each other, and in this embodiment, the polarity of the applied voltage is selected so that the Schottky junction is in a reverse bias state.

FIG. 2 is a diagram showing a photoconductive element according to an embodiment of the structure in which a high impurity concentration semiconductor region is inserted between a semiconductor thin film and an electrode. FIG. 2 is a diagram illustrating a photoconductive element that is the same but whose element formation is easier.

In this embodiment, after forming a multilayer film, a first n (or p) type semiconductor region 8a and a second n-type semiconductor region 8a are formed on one opposing wall surface of the multilayer film using selective ion implantation.
(or p) type semiconductor 1Disi8b and a p+ (or n) type semiconductor region 9 on one side of the other opposing wall surface of the multilayer film.

At this time, the n- (or p-) type semiconductor thin film 5 and the first n-
+ (or p'') type semiconductor region 8a, n- (or p-
) type semiconductor thin film 5, second n+ (or p") type semiconductor region 8b, p (or n) type semiconductor thin film # film 6 and p
Since the + (or + > type semiconductor regions 9 have the same conductivity type), they become electrically conductive.

On the other hand, when the conductivity types are different, a pn junction is formed, so that an insulating state can be achieved by selecting the polarity of the applied voltage.

In this way, the photoconductive element of the embodiment shown in FIG. 2 uses the ion implantation method, so the first electrode 2, the second electrode 3,
The third electrode 4 can be formed on the surface of the substrate 1, making the device much easier to manufacture than the device shown in FIG.

The n- (or p) type semiconductor thin film 5 has a dark conductor -8'
-/ Use a material with low electrical conductivity (σ-103ycm).

Therefore, when no light is irradiated, the impedance value between the first electrode 2 and the second electrode 3 is almost infinite. However, the forbidden band of the n- (or p-) type semiconductor material in the multilayer film When irradiated with light that exceeds the amount of energy determined by the width, carriers consisting of electron-hole pairs are generated in the semiconductor, and a finite impedance exists between the first electrode 2 and the second electrode 3. . Change in impedance at this time ■
is e1ΔF(τn ′μn+τp 0μp) no W
Determined by tube d.

Here, e is the electric charge m (q), F is the generation rate of carriers per second (cm/5ee), τn and τp are the lifetimes of electrons and holes (sec), and μn and μp are the lifetimes of electrons and holes. Mobility (cm/v, 5ec), n is the number of layers in the multilayer film, L and W are the distance between the first electrode 2 and the second electrode 3 and the electrode width (cm), d is n-(or p-) type semiconductor thin film-brow thickness (cm), A is a coefficient related to absorption of light passing through the multilayer film portion.

In the field of high-speed electronic measurement, when using this photoconductive element, the output electrical signal contains high frequency components (ultra-short pulse), so distributed constant circuits with specific impedance such as microstrip lines are used. is often used.

Therefore, in such usage conditions, it is necessary to take impedance matching between the impedance of the element and the circuit when irradiated with light. Therefore, the number n of layers in the multilayer film must be determined in consideration of two limiting conditions: photosensitivity characteristics and impedance characteristics.

+< or n ) type semiconductor thin film 6 has a lifetime τ of minority carriers among carriers generated by light irradiation.
This is to improve p.

At the time when the light irradiation ends, electrons with higher mobility recombine with holes supplied from the first electrode 2 and disappear. As a result, the minority carriers left behind in the n (or p''''') type semiconductor lose their recombination partner and have a long lifetime τp, which increases the high-speed response of the photoconductive element. This can be prevented. Therefore,
n- (or p-) type semiconductor thin film 5 to reduce τp
A p+ (or +) type semiconductor thin film 6 is formed adjacent to the p+ (or +) type semiconductor thin film 6.

A high electric field is generated in the depletion layer between the p-n junctions of both thin films 5.6, and when minority carriers drift into the high electric field, the minority carriers are p + ( or n+) side, and n- (or p-
> quickly removed from the shaped semiconductor. The drift distance of a minority carrier strongly depends on the mean free path length of that carrier.

The mean free path in this case is determined by the following formula, and is closely related to the mobility μ of the material.

Here, m is the mass of the carrier (g), k is Boltzmann's constant (J/'K), and T is the absolute temperature ('K).

Considering this problem, the minority carriers that inhibit high-speed response are
In order to absorb into the n- (or p-) type semiconductor thin film 6 with high probability, the maximum thickness d of the n- (or p-) type semiconductor thin film 6 is as follows.
It is desirable to have at most 300 people.

An embodiment related to high-speed response of the present invention has been described above, and next, an embodiment related to wavelength sensitivity of light will be described.

When the n- (or p-> type semiconductor thin film 5, which is a photoconductive material, is formed of one type of semiconductor, the sensitivity to the wavelength of light is
It shows a peak value in the wavelength band of intrinsic photoconductivity determined by the forbidden band width of the material, and decreases sharply on both the long wavelength side and the short wavelength side.

Therefore, the n- (or p-) type semiconductor thin film 5 is formed of a mixed crystal of a first substance and a second substance having different wavelength bands of specific photoconductivity. If an n- (or p-) type semiconductor is formed with this mixed crystal ratio, it is possible to freely control the wavelength sensitivity spectrum within the wavelength band of the first material and the second material. Next, the case of a multilayer film in which the mixed crystal ratio is gradually changed from the substrate toward the surface will be described.

FIG. 3 is a diagram showing details of a multilayer film portion made of a mixed crystal semiconductor. For example, silicon (Si),
In Figure 0, which describes an example of a multilayer film when germanium (Ge) is selected as the second material, l is the substrate, a is the multilayer film, and b is n- (or p->
C represents a p (or n+) type semiconductor, d represents silicon (Si), and f represents germanium (Ge).

As shown in the figure, the state changes from a germanium (Ge) rich state on the substrate side to a silicon (Si) rich state as it approaches the surface side. This neglects the fact that the forbidden band width of the n- (or p-) type semiconductor thin film 5 gradually increases from the substrate 1 toward the front surface. That is, the structure is such that the surface side is sensitive to shorter wavelength light, and the substrate side is sensitive to longer wavelength light.

Now, if we consider the absorption characteristics of light into semiconductor materials,
Light with short wavelengths is easily absorbed near the surface of the semiconductor, while light with long wavelengths reaches deep inside the semiconductor.

Therefore, when determining the mixed crystal ratio by taking into account the inherent photoconductive properties and light absorption properties of the semiconductor, it is found that the peak spectrum of the inherent photoconductivity of the two substances (the first substance and the second substance) is sandwiched between each other. This makes it possible to form a multilayer film with sensitivity characteristics that are wavelength-independent in the wavelength range.

We have explained that light enters the substrate 1 from the front side, but if the substrate 1 is made of a material that is transparent to light within a certain range of wavelengths, the light will enter the substrate 1 from the back side. However, there is no particular problem.

In this case, a multilayer film without wavelength dependence is formed by reversing the arrangement of the first material and the second material.

Effects> As explained above, the photoconductive element according to the present invention has the following unique effects.

(1) Considering the average free path length of minority carriers, the thickness of the n- (or p-) type semiconductor thin film with photoconductive properties is set to 300 at most. The probability of capturing minority carriers is increased, and this effect can reduce the response time of the photoconductive element to about 10 psec.

Furthermore, it is possible to manufacture an element without deterioration of characteristics.

(3) Materials with high μ can be used, and a photoconductive element with good sensitivity (determined by the change in conductivity Δσ upon irradiation with light) can be manufactured.

(4) The wavelength sensitivity spectrum can be controlled by forming the multilayer film with a mixed crystal.

(5) The wavelength dependence of sensitivity can be improved by changing the mixed crystal ratio in the thickness direction.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a photoconductive element according to an embodiment of the present invention having a multilayer film structure, and FIG. 2 is a diagram showing an embodiment of a structure in which a high impurity concentration semiconductor region is inserted between a semiconductor thin film and an electrode. FIG. 3, a diagram showing a photoconductive element, is a diagram showing details of a multilayer film portion made of a mixed crystal semiconductor. In the figure, l is the substrate, 2 is the first electrode, 3 is the second electrode, 4 is the third electrode, 5 is +< or p) type semiconductor iJ IIQ, 6
is a p'' (or n) type semiconductor thin film 7 is an ohmic region, 8
a is the first n (or p) type semiconductor region, 8b is the second n+ (or p+) type semiconductor region, and 9 is the p+ (or n+) type semiconductor region.
Each hairstyle semiconductor region is shown. Patent Applicant Anritsu Corporation Agent Patent Attorney Koike Engraving of thin and thick drawings (no changes in content) 1...Substrate 2...First electrode 3...Second electrode 4...No. 3 electrode 5...n- (or p-) type semiconductor thin film 6...p+(
or n'') type semiconductor thin film 7... Ohmic region 1... Substrate Figure 2 2... First electrode 3... Second electrode 4... Third electrode 5...・n″″ (or p-) type semiconductor thin film 6...p+
(or ♂) type semiconductor thin film 8a...first n゛ (or inner type semiconductor region 8b...
Second n+ (or p'') type semiconductor region 9... da (or rL+) type semiconductor device 1... substrate a... multilayer film b... n- (or p-) type semiconductor C. ...pτ (or n4) type semiconductor d...St (silicon) e -S io,s Ge a,s (mixed crystal ratio) f
...C, e (germanium) Procedural amendment written form) January/Date of 1985 Patent Application No. 180990 2, Name of invention Photoconductive element 3, Address of person making amendment ■106 Tokyo 5-10-27-5 Minami-Azabu, Miyako-ku
, Date of amendment order (shipment address) October 8, 1985 (October 28, 1986) 6
,? ! Positive target drawing (all drawings) 7.? ! Correct contents Figures 1, 2, and 3 are corrected as shown in the attached sheet. It is clearly drawn in black.

Claims (10)

[Claims] (1) A substrate (1); an n^- (or p^-) type semiconductor thin film (5) formed on the substrate (1) and having two or more photoconductive layers; ) and one layer of p^+ (or n^+) type semiconductor thin film (6) formed between the two or more layers of n^- (or p^-) type semiconductor thin film ( 5) a first electrode (2) electrically connected to
and a second electrode (3); and a third electrode (4) electrically connected to all of the p^+ (or n^+) type semiconductor thin film (6). (2) At least one of the first electrode (2) and the second electrode (3) and the two or more layers of n^- (or p^
2. The photoconductive element according to claim 1, wherein the photoconductive element is electrically connected to the -) type semiconductor thin film (5) by an ohmic region. (3) n^- (or p^) formed on the substrate (1)
-) type semiconductor thin film (5) has a thickness of at most 300 Å.
A photoconductive element according to claim 1, characterized in that the photoconductive element does not exceed the scope of the present invention. (4) At least one of the first electrode (2) and the second electrode (3) and the two or more layers of n^- (or p^
-) type semiconductor thin film (5) is electrically connected to the n^+ (or p^+) type semiconductor region (8). . (5) The photoconductive element according to claim 1, wherein the substrate (1) is a transparent body having light transmittance. (6) a substrate (1); an n^- (or p^-) type semiconductor thin film (5) formed on the substrate (1) and having two or more photoconductive layers; ) and one layer of p^+ (or n^+) type semiconductor thin film (6) formed between the two or more layers of n^- (or p^-) type semiconductor thin film ( 5) a first electrode (2) electrically connected to
and a second electrode (3); and a third electrode (4) electrically connected to all of the p^+ (or n^+) type semiconductor thin film (6); (or p^-) type semiconductor thin film (5) is a multilayer film made of a mixed crystal of a first substance and a second substance, and is formed from the substrate (1) side above the surface of the substrate (1). A photoconductive element characterized in that the mixed crystal ratio of the first substance and the second substance is changed so that the sensitivity gradually changes as the wavelength becomes shorter. (7) At least one of the first electrode (2) and the second electrode (3) and the two or more layers of n^- (or p^
7. The photoconductive element according to claim 6, wherein the photoconductive element is electrically connected to the -) type semiconductor thin film (5) by an ohmic region. (8) n^- (or p^) formed on the substrate (1)
-) type semiconductor thin film (5) has a thickness of at most 300 Å.
The photoconductive element according to claim 6, characterized in that the photoconductive element does not exceed . (9) At least one of the first electrode (2) and the second electrode (3) and the two or more layers of n^- (or p^
-) type semiconductor thin film (5) is electrically connected to the n^+ (or p^+) type semiconductor region (8). . (10) The photoconductive element according to claim 6, wherein the first substance is silicon and the second substance is germanium.